I can't watch the videos but if it's a turbo car they could be brake boosting to build pressure for when they let off the brake fully. Also pressing the brake to slow the car while keeping your foot on the gas would allow you to not have any delay in getting back on the gas at corner exit. I guess tricking the LSD is one use of it also.
Brake Boost Technique: Gradually apply the brakes, as you gradually apply more gas pedal......the trick is to maintain the same mph while rolling (say 40mph), while gradually adding more gas pedal, in turn creating a load on the motor as it loads against the drivetrain. This will in turn start spooling the turbo(s). Once you have a desired amount of boost built up, you release the brakes and mash the gas.