Motorbikes, like pedal bikes, tend to use a chain to transfer power from the engine to the rear wheel. However they’re usually (but not always) slightly more powerful than the average human, so the chain is often the weak point. Plus it can eat trouser legs and flick oil all over the place, thus the shaft-drive was developed.
Working in the same way a car’s driveline does, the chain is replaced by a rotating shaft and a gear assembly, which makes a shaft-drive more expensive and heavier than a chain, but better in pretty much every other respect. Plus it sounds a bit rude.
Flickr’s František Hajdekr has chosen the latter option for his Technic BMW-esque motorcycle, a brand that has used shaft-drive designs for much of their range (including the R 1200 GS Adventure immortalised in the ace 42063 Technic set). Working steering, rear suspension, and a seat made from Batman’s chest also feature, and you can see more of František’s shaft-driven bike at his photostream via the link.