Uuuh, got it backwards
Line locks kind-of do the opposite to what everyone has said...
You use your brakes like normal at all times. But, when you want to do a launch and keep from rolling, with your foot already PRESSING the brakes, you hold the line-lock button. This locks the pressurized fluid (from your foot-pressure) already holding the calipers into your rotors from returning to your master cylinder. Then, when you release the push-button (which is supplying 12V to the solenoid that is CLOSED (hydraulically), the solenoid allows the pressurized hydraulic fluid to return from your calipers to your master cylinder.