Launch control and a line-locker are two totally seperate devices AFAIK.
I know little about launch control although was under the assumption it interacts with your transmission and is possibly some form of stall-converter on auto or limits wheelspin (feel free to correct).
Line-locker on the other hand is as old as the hills and performs as follows: 1) driver presses brake pedal which in turn sends fluid to both front AND rear brakes thus engaging the caliper, 2) driver presses line-locker button (generally located on steering wheel on drag cars) and then releases brake pedal, 3) brake fluid remains LOCKED in front brakes thus keeping front calipers locked but rears unlocked, 4) driver warms tyres and engine via means of burnout as front tyres are locked (only useful in RWD obviously).
Watch Jap drag footage such as GT-R class and everybody rips the line-locker and bags the tyres prior to racing.
