In page #37 of the sppedlite 430exII manual, it says : Features available with type B cameras: Manual flash, stroboscopic flash with wireless flash" What the book means by "stroboscopic flash" Isn't flash always stroboscopic?
Canon's top-end speedlites (580EX, 580EX II, 600EX RT) have a "stroboscopic flash" mode (indicated as "MULTI" on the control panel) that fires the flash multiple times at a rate of your choice, giving you multiple flash exposures in a single shot.
You're correct that flash is often synonymous with strobe, especially as used in general photography, but flash isn't necessarily stroboscopic. Stroboscopes fire strobe lights many times at a regular intervals and are often used to reveal periodic behavior. I'm not sure that speedlites are ever used quite like stroboscopes, but the "stroboscopic flash" feature nevertheless resembles a stroboscope.
The 430EX II doesn't have the stroboscopic flash feature when used alone, but it does support the feature when used in slave mode with a master that has the feature. Set your 430EX II to slave mode, set a capable speedlite to master mode and set it up for MULTI shooting, and fire away. (The 580EX manual recommends not firing more than 10 stroboscopic bursts before giving the units a 10 minute rest to let them cool down.)
The stroboscopic feature of the 430EX II is barely documented -- the line you found is about the only hint I can find in the manual that it even exists. Syl Arena mentions it in a tiny "Speedliter's Tip" sidebar on page 349 of his Speedliter's Handbook, and there's also some good information on the web, such as this thread and a hint in DPReview's coverage of the 430EX (apparently, the feature is available in the 430EX as well as the updated 430EX II). Even so, it's easy to overlook multi flash on the 430 models, and it's not unreasonable to think that maybe the unit only appears to support multi flash mode, but is in fact simply being triggered for a single flash each time the master flashes. That is emphatically not the case, and it's easy to verify. Do this:
Set a capable master unit to MULTI mode. Configure it for several flashes at a slow rate, perhaps 6 flashes at 3Hz, so that you can easily count the flashes.
Press the ZOOM button on the master until the Ratio indicator blinks. Turn the jog wheel to set the flash to "FF" and the press the set button. This disables the master, so that the only flash it will emit is the control flash.
Set one or more 430EX or 430EX II units to slave mode.
With the master visible to the slave(s), press the PILOT button on the master.
Observe that the master emits one flash (the control pre-flash), while the slave(s) fire all 6 flashes at 3Hz (or however many you set at whatever rate you set).
As always with Canon's optical wireless system, the control pre-flash fires before the shutter opens and therefore doesn't change the lighting in the shot.
When Canon refers to stroboscopic flash, it is in the context of multiple flashes produced during a single exposure at shutter speeds slower than the camera's Flash Sync Speed. This technique, also referred to as MULTI Flash, is not available with the 430EX II unless it is used as a Slave flash to a Master flash that has this feature. Such master units would include the 580EX II (see page 46 of the 580EX II Instruction Manual), and the 600EX-RT. Unfortunately, other than the hint on page 37 that this question addresses, this capability when used as a slave unit is not documented anywhere in the User Manual or promotional materials for the 430EX II!
Incidentally, Canon refers to multiple flashes used to expose the entire film/sensor plane at shutter speeds above the camera's Flash Sync Speed as High-Speed Sync. This feature is available when using the 430EX II.
A statement similar to the one at the end of the appendix on page 37 of the 430EX II Instruction Manual appears in all of the manuals for Canon flash units that list which Canon E-TTL features work with the older Type B TTL camera bodies and which are only available with the newer Type A E-TTL and E-TTL II bodies. To the best of my knowledge, all Canon Digital SLRs are Type A E-TTL/E-TTL II compatible.