Line lock is a solenoid that hold brake pressure to the front wheels, and not the rears so you can do a burnout without tearing the crap out of your rear brakes, and putting more strain on the drivetrain.
I see that drawing, but I hooked mine up right between the master and proportioning valve. It locked both front tires, although I know a lot of you say it shouldn't LOL. But it worked for me just fine! Makes for some awesome awesome burnouts too.