What to see
In the above demo you see (1) an inner rotating part (the text) placed on (2) a rotating disk made up of coloured dots. The outer disk does not rotate continuously, but ‘swings’ back and forth. What is the motion of the central text? Is is a smooth rotation?
Or doesn’t it rather ‘freeze’ now and then, depending on the outer rotation?
What to do
If you use uncheck the top ‘View’ box it becomes obvious that the central rotation is completely smooth. When the outer disk is visible, it exerts an influence on the perception of the central disk’s rotation.
You can experiment with the speed of the inner and outer parts and the frequency of the swing.
This won the first prize in the 2006 Best Illusion of the Year contest. Max Dürsteler’s lucid comments (→PDF) need no further elaboration.