TL;DR it helps you identify the true diameter of your visual focus, which is said to shrink with old age (mine shrinks more in terms of _time_ dimension but that's a different issue!)

For best results, use it _fullscreen_, change the #define `90` values to a higher value if you're on a high dpi screen.

Stare at a few places on the screen and you'll get the effect of appearing to rotate only where you stare.

It's pretty neat.