What does 'IS start' mean? I use a Canon 80D and located it under Custom Controls>DOF preview button>IS start. Can't seem to locate anything in the manual.
The image is of a shaking hand — I and S stand for "image stabilization".
This is explained on page 439 of the manual (in English):
If you press the button assigned to this function when the lens’s IS switch is set to
<ON>, the lens’s Image Stabilizer will operate.
If you don't know what the initials stand for, or can't guess what the icon might mean, a search for the phrase "IS Start" in in the manual found on Canon's support page for the 80D will jump right to it.
IS stands for Image Stabilization.
For Canon cameras/lenses, IS is normally activated anytime the metering is active and the viewfinder displays information. Most of the time this is accomplished by a half press of the shutter button. IS also activates along with metering when using back button focus. When metering initiated by any of several buttons/actions times out and the viewfinder stops displaying information the IS also times out. This helps to extend battery life by not using power to spin the gyros and operate the IS mechanisms when the camera is not being used to compose or take photos.
There are a few Canon lenses with a 'Mode 3' IS setting that only applies IS correction when the photo is actually taken. In IS 'Mode 3" the IS gyros are spinning but the unit is not providing correction during metering/AF. If one wants to temporarily apply IS correction during composition/metering/AF while using an IS lens in 'Mode 3', there needs to be an additional button to do that. Hence the option to remap the 'DoF Preview' button to 'IS Start'.