Here it feels like you're looking at spirals, but you're not, they're just circles... tricky!
What: The real name for this illusion is the intertwining illusion, which was documented by Dr. Baingio Pinna in 2002 , is the first of Pinna’s illusions that I’ve re-coded up in optical toys, and is my personal favorite. If you hadn’t picked up on it already the illusion is a series of concentric circles, yes concentric, and is not at all a spiral.
How To: There are some neat things to play with here, primarily you can change the angles to see when the illusion is strongest for you and when it breaks completely… The same goes for the colors of the squares and background, you’ll notice that the contrasts are key to making it stronger!
Explain it: If you played around with the colors in the customization tools you would see that the contrast is part of the key here, while I couldn’t access the cited paper, some perusing  has told me that this is your peripheral vision (where the illusion really gets you) is tracking the angles of the squares and is guessing that the shapes are spirals, which then causes you to perceive it this way, even with the knowledge that they’re not.
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 email@example.com, be kind!