Typically the cars have what is called a "roll control" It allows the driver to step on the brakes, push a button and release the pedal. The front brakes remain locked while the back brakes are free'd up, allowing the car to do the burn out. This can also be done by cranking the brake bias all the way to the front brakes but you don't typically see this in drag cars but more in the roundy-rounds. Hope this helps.
Tj