Below you see a circle of purplish colored patches, one of which briefly disappears, circling around.
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 color 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!
EDIT: I made this entry public, if anyone wants to link to it.