Stare at the center of this effect for 20-30 seconds and then look at something else, you'll see the effect transfer.
What: This is the second optical toy on this site demonstrating this effect, although the other is using spirals they have a similar effect, albeit a little more spinny. A motion aftereffect occurs when you observe a moving pattern for a while, and then begin to look at something else.
How To: Stare at the center for a while, and then look at something nearby, be it the wall, art, or your own hands… you should see them appear to bulge and grow, or shrink and contract.
You can also close a single eye, and experience it with … well, half of your site!
Explain It: Again, it’s tricky to explain clearly, but here’s a great encounter  described by Aristotle himself!
also when persons turn away from looking at objects in motion, e.g. rivers, and especially those which flow very rapidly, they find that the visual stimulations still present themselves, for the things really at rest are then seen moving
Ultimately it is believed that your neurons are coding a particular movement, and reduce their responses with time of exposure to a constantly moving stimulus; and when you look away, the neurons that are coding the opposite movement are more active, and hence you see the inverse.
I've researched these illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via firstname.lastname@example.org, be kind!