Fluid Dynamics Lesson (Or Long Winded Rant): The main ingredient here is conservation of energy or, more specifically, the Bernoulli Effect. The blades act like a reaching sail in that they are not going directly downwind. They are off the wind and, thus, the apparent wind relative to the moving blade is much higher than the wind relative to the car. But why does that matter? It is the same reason we can sail upwind. When sailing upwind we are going against the wind and yet we move forward. How? Conservation of energy. The sail shape is such that the velocity of the air moving over the leeward side of the sail is higher than that on the windward side. The faster the air moves the lower the air pressure against the sail (this is the conservation of energy tradeoff) and, thus, the pressure on the windward side, where the airspeed is slower, is higher than that on the leeward side. This net difference is what drives the boat (or blade) forward.