Redditor hallofwindows explains:

Mech. Eng. student here. both links "AC" and "BD" are of equal length resulting in this figure eight pattern. This is known as a "change point", where the coupling link between "AC" and "BD" could go one of two ways. It needs to be initiated into one of two directions.

If one of "AC" or "BD" were shorter than the other, you would get what is known as a "crank-rocker" mechanism, where either "AC" or "BD" could rotate about its fixed point, and the other link would "rock" back and forth. The other would be "rocker-rocker" where neither "AC" or "BD" would be able to rotate about its fixed point, resulting in a "window wiper" type of motion.

Spirograph!

http://www.mathplayground.com/Spiromath.html

Circle, square, hexagon: