My agency trains that the light is only used for target illumination, and is pointed only when a possible use of lethal force is legally and ethically justified. The light should be activated by the non-firing hand. In a fight, you would probably illuminate the target, engage, then immediately turn off the light to minimize the exposure of giving away your position (if possible, moving to a new position quickly to avoid return fire). Sometimes, though, you would continue to illuminate the target (say, taking someone into custody).
Also, remember it is not an aiming light and is not zeroed to the handgun: Use your sights (or at least a flash front sight picture)!
I actually don't use one, because I figure I need to have my flashlight out anyway for searching, etc.. I like tac lights on long guns, though.
Last edited by R1145; January 15, 2013 at 12:07 AM.