Joss,
The carbon prop shaft weighs about a third of the normal one. This reduction in weight means that the engine has less work to do, and less torque to waist, in spinning the prop shaft. The result is very similar to having a lighter flywheel, revs pile on faster, engine spins more freely ...and the revs drop off quiker too.
The 400Rs had one as standard, and Shin has one fitted to his black r34.