You can get hurst line locks pretty cheap if you shop around. Just the risk of getting stuff from places you've never dealt with before and waiting for them to from come over the pond, then tax/duty etc. Whereas you know the craic with the DW guys, you pay, it arrives real quick, sometimes with free food.
They basically lock off a brake line, the manual one is a lever, basically a tap, the electronic ones are are solenoid that opens and closes a valve, in essence they do the exact same thing but for the stuff I'd be trying you want them to work instantly at the easy press of a button, rather than reaching around trying to find a lever and swing it all the way open/closed quickly.
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.
Having a line lock on the front is also good for momentarily turning your footbrake into a hydraulic handbrake.
Its just a plaything really, but could be useful for drifting if you are a good enough driver to be able to exploit it.
I want a pair just for stunting/displays. I Have ever since I found out about them but other things have come first in the list of mods.