Optical Illusion that ROCKS.
Let your gaze rest on the central fixation cross, but observe with your “inner eye” the patches just when they disappear. With good fixation, you should see a strong greenish colour whenever the violet patch has disappeared.
When you are fixating well, after a few cycles you will actually see a rotating green spot! If your gaze is really steady, the magenta patches will disappear, leaving only a rotating green spot!
That rules.
Stolen from http://www.michaelbach.de/ot/col_rapidAfterimage/index.html