A line lock (or roll control) is an electric solenoid, usually inserted in the brake line to the front wheels. You press hard on the brakes, engage the solenoid. The front brakes are then still locked, as long as you keep the button pressed, the rears are free to spin without fighting the brakes. Most can do a burn out for as long as they want , the car won't move however.
You can experiment with brake pressure, to allow the car to move, slowly, while holding the line lock. That's how they do the looong smoky burnouts.
