This is a type of optical illusion called a "peripheral drift illusion", similar to one originally designed by Akiyoshi Kitaoka. The idea is that the squares appear to be turning slowly in on themselves. Especially if you focus on things *next to* the image, like the edge of the computer screen.
Akiyoshi Kitaoka has an original on his website, but he has so many illusions that it is difficult to find this one again!
