The red cogwheel rotates, but does it rotate smoothely? Physically, the rotation is perfectly smooth, as you can confirm by making parts of the arrangement transparent, using the sliders. However it seemingly jolts ever so often. Depending on conditions (contrast to the background, and between the stationary and rotating part) other percepts arise: e.g., a backwards jerk of the central wheel when everthing fits.
You can select between 3 different presets or adjust colours, transparency, and motion speed.
This effect shows that there are still new illusions out there to be discovered and/or formally described. As Peter van der Helm discusses in his paper (2007), the effects here are probably caused by colour assimilation and/or ambiguous figure-ground segregation.
van der Helm PA (2007) Kaleidoscopic motion and velocity illusions. Vision Res 47:460–465 [PDF]
Peter van der Helm’s own demonstration page