Its really a kind of screw gear set up. The cable is attached to a gear which is functionally in line with the cable - kind of like it is wrapped around the cable (although I bet its just bonded in some way). That gear engages a gear on the wiper shaft pivot. So, the cable turns one way and it turns the gear in one direction which moves the wiper. It reverses the direction and turns it in the other direction. Its like a driveshaft (and pinion) attaching to the ring gear. The driveshaft is the cable. The pinion is the gear at the end of the cable and the ring gear is the gear on the wiper shaft.

Having it on a cable just allows it to be routed around curves.

Make sense?