Comment by sircastor

It’s not clear from the picture. The belt seen is actually on one of three pulleys. The pulley on the left of the belt drives shaft b, which spins B and turns the output. Likewise, the puller to the rights of the belt will turn a, and therefore A.

The things that are most unclear is that the shaft between b and B is hollow (allowing the shaft from a to A to pass through) and that there are three unconnected pulleys the belt can ride on.