it opens a extra exhuast valve which in turn stays open and lets all the pressure from the piston going up out. which like you said slow vehicle.thers more then one type actually.
I used to have a cop loudspeaker hooked up to my CB. When I'd catch someone drifting, I'd shout out "OH SHIT NO BRAKES" and watch them floor it and straighten up, looking around to figure out where the voice came from.