Screenshots from Video, page-57

When the movement of the wheel spoke (or in this case, propeller rotation) synchronizes with the frame rate, the rotation is seen as static, or moving in the other direction. This video was taken with a very short shutter speed, which kept the blades from blurring, and a frame rate that synchronized with the blade motion. Unless you can vary the frame rate while using the camera (which you can't), capturing this effect is purely by chance. "