bike swinging is more useful at lower RPMs. almost everybody gets out of the saddle and rocks the bike to start a sprint, both on the track and on the road. on the road, people can shift, keep their cadence low, in the rocking zone, and rock all the way to the line - but on the track, once you rock to accelerate, most people drop into the saddle and finish at a cadence that's too high to take advantage of rocking.