I think what happens is that the wires are fixed length, but wrapped around a ring on the back axle. As the pedal is pushed down, it moves the pulley, unwrappi

I think what happens is that the wires are fixed length, but wrapped around a ring on the back axle. As the pedal is pushed down, it moves the pulley, unwrapping the wire from the ring, which rotates the rear wheel through a ratcheting mechanism. I'm not sure what causes the wire to rewrap itself around the ring on the upstroke... probably some sort of embedded spring.