I built this vertical axis wind turbine with blades that can rotate freely. The wind causes the blades to swing to a closed position to catch/block more wind. As the closed blades rotate 180 degrees, they are now going against oncoming wind. The wind causes the blades to open such that the wind can move through and around the blades, so there is less resistance to oncoming wind.

My prototype doesn’t work well. I think the blades are heavy and too wide for the size of the turbine, so the moment of inertia is too high for the blades to close fast enough, and there is too much friction to turn the blades.