[pb060]

I don’t understand why you need the animation. If you point at the green background after staring the red one for a while, you can see a whole circle of the saturated color.

[spragl]

I agree, that you dont. As far as I can tell, the effect comes from superimposing an after-image on some other image.

[zahlman]

You clearly demonstrated that you don't need the animation. However, it's a convenient way to keep you staring in the same place and not have to do anything else to cause the effect.