I am trying to understand how wireless external flash works with Canon EOS M6. Manual didn't help, and the specifications just say "wireless multi-flash support". What does that mean?
As far as I understand theres two different modes - one is infrared/flash control where the built-in flash controls other flashes. Is this visible to the naked eye? Does this actually fire the flash at the time of the photo (enough to light)?
The second one is radio controlled. This needs a transmitter, right?
... the specifications just say "wireless multi-flash support". What does that mean?
It's obscure, but basically what that spec means is that the M6 can only do Canon's wireless flash systems (both radio and "smart" optical) via the flash hotshoe and camera menus, but not with the pop-up flash. You have no built-in "master" unit to trigger an off-camera flash, you need additional external flash units (i.e., "multiple flash units": one on camera to act as master, and one or more off-camera flashes to act as slaves).
As far as I understand theres two different modes - one is infrared/flash control where the built-in flash controls other flashes. Is this visible to the naked eye?
Actually it's near-infrared, and the signaling is still in the visible spectrum. :) But you probably won't see it, as it's very quick. And as I said above, the M6's pop-up flash does not have this capability. You need a higher-end dRebel or prosumer dSLR body to have this function in the pop-up flash. The EOS M, SL, and lower-end (XXXXD) dRebel bodies do not have this capability. To use this system, you'd need a "smart" optical master unit on the hotshoe, such as an ST-E2, or a 580EXII (see: Is there a Canon equivalent of the Nikon CLS?)
Does this actually fire the [popup] flash at the time of the photo (enough to light)? ... Can I have the 'master flash' (even if its not the built in one) NOT light the scene, but only signal to other flashes?
If your camera had the Canon "smart" wireless master capability, you can set the "master" unit not to fire at the time of the main burst. The flash, however, will actually fire (as the signal to the remotes to fire), only at a low enough power setting that, at non-macro subject distances, its light should not appear in the image. But your camera doesn't have this capability.
The only optical slaving system you can use with the M6's popup flash is a "dumb" optical system. This is often called S1/S2 mode on some 3rd party flashes (Canon's own speedlites cannot do this). This type of mode is manual only (can only fire the flash, no settings controls), and simply uses a sensor on the flash to tell the flash to fire when it "sees" another flash burst. S1 will fire on the first burst sensed; S2 on the second burst (thereby skipping a TTL metering pre-flash). If you are using one of these modes, then the pop-up flash will have to fire brightly enough to set off the optical slave, and that may register in the image.
The second one is radio controlled. This needs a transmitter, right?
Yes. This is true of any radio flash triggering system, not just Canon's RT system (e.g., Godox's X, Nissin's Air, Cactus's V6, Yongnuo's YN-560/622/RT, or Phottix's Ares triggers, etc. etc). You need a transmitter unit on the hotshoe of the camera to act as a master, as well as receivers either attached to or built-into the off-camera "slave" flashes.
See also:
In order for the EOS M6 to control a flash via E-TTL, an E-TTL compatible "master" flash or transmitter needs to be physically connected to the hot shoe either directly or via an off-shoe cable or external wireless trigger that is E-TTL compatible. Until the release of the 600EX-RT in 2012 the Canon wireless system was an optically controlled one that uses pre-flashes to meter and communicate with any off camera flashes. Most recent bodies with built-in flash (beginning with the 7D in 2009) can control off-camera slave flashes optically using the built-in flash, but lower end entry level EOS DSLRs and the compact EOS M series, some of which don't even have built-in flashes, can not. No Canon EOS bodies made earlier than 2009 has a built-in master flash. No Canon full frame digital body or 1-series APS-H digital body has ever had a built-in flash.
Here is a chart that outlines how the optical control process works.
Does this actually fire the flash at the time of the photo (enough to light)?
The built-in "master" flash on many EOS cameras with 'master' capability can be set to only fire the pre-flashes during communication prior to the shutter opening. The preflash communication tells each of the slave flashes exactly how long to wait before firing their main flashes, and the camera times the shutter release to coincide without firing the built-in or shoe mounted 'master' flash again. In theory the light from the "master" flash set to 'Remote Flash(es) only' should not influence the appearance of the image taken when the shutter is open. In practice it sometimes does because the tube on the 'master' flash is still dispersing the energy remaining inside the bulb from the pre-flash communication pulses when the shutter opens.
Here's what the item in the EOS 7D Mark II menu looks like. The 'Remote flash only' setting is circled in yellow.
Unfortunately, the EOS M6's built-in flash does not have 'master' capability.
When using an external optical 'master' flash on the camera's hot shoe, the 'master' flash might be capable of being set using the 'wireless function' setting to control the other flashes via pre-flash pulses without firing a main flash during exposure. This is much the same as the option for an internal flash shown above. This seems to depend in the menu options for each camera model and the capabilities of each flash model. My EOS 5D Mark II has the option of setting the 'master' flash to 'disabled' from within the 'wireless function' sub-menu (Note: This a different menu item inside the 'wireless function' sub-menu, in the main 'Flash function settings' one tier up flash must be set to 'enabled' to fire the pre-flashes used to control the off camera slaves). This sub-menu is not covered in line-by-line detail in the EOS 5D Mark II Instruction Manual. (I doubt it's covered in the EOS M6 manual either, but at the time this answer is being written the downloadable PDF version of the manual is blank past page five when downloaded from any of Canon's global region support sites.)
For optical control either an optical 'Master Capable Flash' or optical 'Master Transmitter' must be used on the EOS M6 hot shoe. Any E-TTL capable 'slave' flash can receive the optical signals from the 'master'. Any Canon flash with 'EX' in the model name can receive optical E-TTL communication from an optical 'master' flash. There are also many third party flashes that are capable of being E-TTL 'masters' and/or 'slaves'.
Until the 600EX-RT and other Canon 'RT' (radio transmission) flashes came along, the only option for radio control of a Canon E-TTL flash was to use a third party wireless trigger system that is E-TTL compatible and allows communication in both directions between the camera and flash unit. You need an E-TTL capable radio transmitter attached to the camera's hot shoe and a compatible receiver attached to the flash's hot foot. Various radio systems from different makers aren't compatible with each other, so the transmitter and receiver need to use the same radio protocol. Several third party brands even have more than one radio protocol for various product lines.
There are also several third party makers that sell flashes compatible with the Canon RT system. The ST-E3-RT wireless transmitter is only capable of radio wireless control (Note: the Canon ST-E3-RT does not have a near-infrared AF assist lamp, but some third party clones, like the Yongnuo YN-E3-RT, do). The older ST-E2 wireless transmitter is only capable of optical control (and includes a near-infrared AF assist lamp). The radio capable Canon RT 'master' flashes can be set to control the off-camera flashes via radio without firing their own flash tubes.
Both Canon and third party flashes and wireless controllers on the 'RT' system should be compatible. Most third party RT products seem to work well with Canon RT products and vice versa, but some third party RT products don't work as well with other RT products from different third party makers.
The 600EX-RT and other RT flashes (430EX III-RT, 600EX II-RT, some third party 'RT' flashes) that now have radio capability also have optical slave capability when paired with any other optical E-TTL 'master' capable product. The 430EX III-RT radio is both master and slave capable, but is only capable as a slave using the older optical control system. The 600EX-RT and 600EX II-RT can be both master or slave in either the optical or radio system.
Beyond that, what functions your camera can control are dependent upon the capabilities of the connected E-TTL flash unit. Since your EOS M6 is a Type-A camera that can use all the features of EX-series Speedlites, it should support any camera controllable E-TTL feature if the attached flash is capable of that feature. If, for example, the external flash is capable of manual output power then power output will be controllable via your camera's menu. If the flash is not capable of manual adjustment, then that function will not be available to you in the camera's menu, and so on.
If the flashes are capable of being used in groups and ratios, then your camera can control them via the menu. With multiple flashes, a 'Master Capable Flash' such as the 580EX II or "Master Transmitter" such as the ST-E2 must be connected via the hot shoe (directly, cable, or wireless trigger). The other flashes can be 'Slave Only' capable. All of the flashes/controllers must be either optical or radio. You can't, for example, control an optical only slave with a radio only master unless the slave has a radio receiver attached to the flash's hot foot.
When Canon first introduced the EOS-M system they also introduced the 90EX flash. It was a compact flash intended for use with the EOS-M cameras that has since apparently been discontinued. Some 'white box' units¹ are still available from some sellers. Even though it is small and has relatively low flash power, it also has full optical master flash capability. A lot of Canon EOS DSLR shooters bought the 90EX as a much more budget friendly alternative to the Canon ST-E2 optical transmitter and used it to to control off-camera E-TTL flashes from the hot shoe of their EOS DSLRs.
Optically controlled remote flash is fast becoming old technology, though. Radio has a number of advantages over optical communication. Among them:
Difficulty with bright ambient light. Especially outside under sunlight, the power of optical wireless control is very limited. Again, especially with a relatively weak built-in popup flash or a near-infrared transmitter, the master just doesn't have much power to cut through the bright sunlight and the receivers can't detect the weak signal from the master over the very bright sunlight. Radios work just as well in bright sunlight as they do in a dark studio.
Multiple Photographers. Radio has the ability for more than one set of the same type to be used in proximity to one another without interfering with each other. (Think several press photographers all using Canon covering an event for multiple publishers. Or more than one shooter at a wedding.)
At this point, the two most attractive options are probably to go with the Canon RT system or with a third party radio system.
The Canon RT system has the advantage of being used by Canon's own radio flashes. Flashes made by the various camera makers for use with their own cameras tend to be the most reliable and don't usually have compatibility issues with newer products introduced later by that same manufacturer. The RT system also has less expensive options available from third party makers that can be mixed and matched with official Canon RT units. So one can buy a "main" Canon flash or two to insure rock-solid reliability and also a few cheaper third party flashes at prices that allow buying a 'backup' or two without breaking the bank and make them more or less disposable if they do break.
A third party radio system (that is not a clone of the RT system) has the disadvantage of not working with Canon's own flashes without adding a compatible receiver to the foot of the Canon flashes. But some third party systems offer more flexibility and also offer cross compatibility between the wireless flash systems of different camera makers. The current favorite is probably the Godox system. Godox products are also marketed under trade names of various sellers. In the U.S., Adorama's 'Flashpoint' products are all rebranded Godox products that are fully cross compatible with Godox branded models.
One nice thing about the Godox 2.4GHz system is that it uses the same transmitter/controller protocol to control manual only flashes, TTL flashes, battery powered 'portable' studio flashes, and full studio flashes that are powered by plugging a cord into a wall socket. As much as that sounds like the obvious way to keep customers loyal to one brand as they grow a flash system from cheap, manual only speedlights all the way to full studio strobes, they're pretty much the only one that does it seamlessly. The other nice thing is that the radio receivers in the Godox TTL flashes can automatically recognize and switch between a "Canon" version of a Godox transmitter attached to a Canon camera and a "Nikon", or "Sony", or "Fuji" version of a Godox transmitter attached to one of those respective brands of camera with full TTL capability within the bounds of each respective system.
¹ "White box" units are things such as lenses, flashes, batteries, chargers, etc. that were bundled by Canon for sale in a kit with a camera body. Sellers sometimes break up the kits and sell each item individually. This might mean that there is no warranty directly from Canon, but only from the seller. YMMV.
