Hi Angus,
The cush drive is and assembly at the rear hub consisting of metal "teeth" for lack of a better word - that sticks into those slots in the rubbers you see in above photo. The rubbers are inserted into the main hub and the teeth or lugs I mentioned are in the part of hub that the rear sprocket mounts to. Next time you remove your rear wheel, give the sprocket a good tug, that part comes out and you can see the rubbers. All they do is soften the delivery of torque to the rear wheel and keep it quiet. All chain and belt drives have them as far as I know. They wear and tear on hard excelleration and decelleration. If they ever fail completely, the lugs will strip through the rubber and slam into the metal parts in the hub and probably break through those. I hope this didn't complicate your understanding. They get weak and make the drive system feel sloppy, like your chain is too loose - Noisy too.
Maybe someone else cares to elaborate on this description.
Ride Safe,
Dennis
