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Movies are shot at 24 frames per second (typically, not counting Maxivision and other uncommon formats). As a wheel (or a propeller) spins up, it will rotate at increasing RPMs. The wheel's rotation rate will have a certain resonance with the 24 fps shooting rate of film. At some resonances, the wheel will appear to be spinning slowly forward. At RPMs closely resonant with the 24 frames, the wheel will appear to be standing still (if blurrily). Going past that rate will cause the appearance of slow backward rotation.

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