A line lock if used with the brakes off will just stop the fluid pressure getting to the brake/s that the line its fitted to feeds, if you use it when the brake pedal is pressed then it will lock the pressure in.
For burnouts you have a line lock fitted to the brake line feeding the rear brakes, lock the line without the brakes pressed, so when you press the pedal it only pressurised the front brakes, leaving the rears free to spin and not burn out your brake friction material too.