As I understand the wind doesn't power the propeller. The wheels do that, they are connected by a bike chain to the prop and there is light gearing there. Once the wind starts pushing the cart, the wheels begin to spin the prop which works like a fan a pushes against the wind. Supposedly this thing can accelerate constantly, since the faster it moves, the faster the prop spins generating a growing force that pushes against the wind. You control the speed by the use of brakes and adjusting the prop pitch.