A line lock is a very nice item to have if you frequent the dragstrip... (1) you don't have to heel-toe the brake and gas pedals to do your burnout (extending the life of your rear brakes) & (2) if you've ever had problems with rolling out past the beams, you can lock your car solidly in place on the starting line... just be sure to time your release of the line lock button with your launch properly, lol.
Here's how I set mine up...
