The intention of Line Lock, which is in the Track Apps suite, is to heat up the tires before a launch at a drag strip and definitely not for hooligans to do on public roads to try and impress their friends before crashing into a line of trees.
What it does is lock the front brakes fully while leaving the rear brakes disengaged. This gives you a perfect smoky burnout every single time, with white smoke and testosterone shooting off the rear tires.
Line Lock is essentially the Mustang GT's mating call, the way it shows dominance over other, lesser pony cars. It can also make a ham fisted moron into a lead footed dynamo.
Oh, and if you want a GIF of this burnout magic, our friends at Road & Track have you covered.