If I'm thinking correctly the idea behind is it uses the engines compression to help slow the car. If you've ever driven a manual transmission car it would be sismilar to what you feel when downshifting rapidly. I don't know if this is used in real life race cars or not, I've just seen it in other LMS mods. In real life, you see or hear it used in semi tractor-trailer type trucks to help in slowing the vehicle in order to keep from overheating the brakes.