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.