I always thought that launch control worked with the thottle pinned to the floor. If your left foot is on the brake, the revs won't go about a certain amount with DBW. So if you are in Sport with the brake pressed and throttle pinned, the car knows you are wanting a rapid start, so will give you the pre-programmed amount of revs to give you a consistant launch every time. If you are modulating the throttle yourselves, surely the car will not recognise that as a request for launch control, and it will be you doing the driving instead.