I'm guessing you mean the launch control mapping available to those on the later 1.8t ECU, in which case it's a second limiter - whatever rpm this is mapped to, mine is set to 4500rpm, which activates when your foot is on the clutch and you're stationary, negative timing is also thrown in. This all keeps the turbo spinning to build boost and hold revs at the pre-set rpm ready for the car to launch off the line.
There's a lot of fireworks that come with it all, detrimental to many components, especially the turbo. I found it absolutely useless on fwd chassis.