baxter:
its just an additional piece that is driven by the crank in the case. looks almost like one of those ducted fan assemblies. i dont really see anything beneficial to the design and may even be more a disadvantage because of the added weight for the crank to carry.
the only reason i know is because my friend took apart a hyper engine before and i saw the assembly. but this was a while ago and the newer hyper engines may have dumped the design.