nothing?
kidding...mechanically it transfers power from the crankshaft to the jackshaft (by means of a single assembly) then on back to the rear sprocket.
the gearbox, imo, is over-engineered...all the assembly does is replace the old-school torque-convertor arrangement. however, it does allow for a true "neutral"...maybe it's only real benefit. again, opinion only.
that's about all i can say because i only have pictures and general experience to work with as of yet...someday i may be able to give an "exploded view" of it's real-time performance, until then i'll have to glean what i can from the 4-stroke happy-time pioneers.