Warning: this is a subtle effect.
What to do
Fixate on the cross, but watch the moving ring. In other words: dissociate gaze direction and attention; this takes some practice.
What to observe
By now you will have noticed that the blue content of the ring is occasionally replaced by a yellow shape. Is it a full yellow disk or a yellow crescent? If you fixate on the cross, you should only see a crescent. If you follow the ring, you see the full disk. TLC (=tender loving cooperation) required ;-) [in other words: the effect can be somewhat subtle].
What else to do
Instead of fixating on the cross, you can follow the ring.
You can try to stop the motion just in time for the yellow disk to appear to convince you it is really a full disk.
You can press the ‘slow’ button to, well, yes: slow down the motion… No illusion occurs any more.
BTW: should the flashing icon at top right get on your nerve (happens for me), simply scroll down the page so the icon just moves off screen.
I programmed the first version of this inspired by a fascinating talk by Romi Nijhawan in Freiburg. He will soon publish a paper on this. It is one of the many demonstrations of the “flash-lag” effect. The explanation, in a nutshell: Our mental perception and planning mechanisms need to take into account the delays in afference, computation & efference. Thus moving objects are “perceived” a bit ahead of their assumed trajectory; the flash (being essentially stationary) is not. Consequently, one perceives a positional disparity between briefly flashed stationary and moving objects.
I used to think that the flash-lag effect is “the same as” MacKay’s stroboscopic illusion, but since it does not need large luminance differences, it is not. Both belong, however, to the general scope of problems to integrate sensory information constrained by sensory delays depending on, e.g. ambient luminance, and intermodal latency differences in the order of decades of milliseconds.
Nijhawan R (1994) Motion extrapolation in catching. Nature 370:256–257
and many follow-ups and discussions, e.g.
Eagleman DM, Sejnowski TJ (2002) Untangling spatial from temporal illusions. TINS 25:293
Krekelberg B, Lappe M (2002) Response: Untangling spatial from temporal illusions. TINS 25:294
Enns JT, Brehaut JC, Shore DI (1999) The duration of a brief event in the mind's eye. J Gen Psychol 126:355–373