I've only heard of brake boosting being done with turbo cars. It involves getting on the gas while at the same time staying on the brake - this way, back pressure is built up through the exhaust, which spools up the turbo and raises boost. Brake boosting is used as a way of eliminating lag inherent with turbos. You can brake boost from both a stop or from a roll, but, essentially, the process (hitting the gas while standing on the brake until you are ready to go) is the same.