Who doesnt love optical illusions? Communists, thats who.
This illusion is called Motion Aftereffect.
"The motion after-effect (MAE) can be explained by adaptation in visual neurons that respond selectively to moving contours in the image. In the absence of image motion, cells tuned to different directions produce roughly equal responses. Exposure to a particular direction of motion alters the balance of activity in favour of cells tuned to the opposite direction, causing the after-effect."
TLDR: its cool, try it and post your favorite optical illusions