When the line lock is not activated it let's the fluid go in either direction, as in press the pedal and if flows thru the lock and activates the brakes. Release the pedal and the fluid is allowed to go back to the master. The line lock is plumbed into the front brake feed. Between the master and front brakes. When you activate the lock it allows fluid to go to the front brakes but not go back to the master. You typically would press the brake pedal to build pressure hit the button and hold it, release the brake pedal-this would keep the front brakes activated while letting the rear brakes release. You can then hit the gas and do your burn out, if it still pushes thru the brakes you can pump the pedal once or twice more and release to build more pressure. The lock now acts like a one way check valve, when activated. The rear brakes are still functional if you use the pedal. To let the car roll again you would release the button and it in turn would let the fluid go back to the master just like normal.
