Above you see a circle of blue-violet (magenta) patches, one of which briefly disappears, circling around.
Let your gaze rest on the central fixation cross, but observe with your “inner eye” the patches just when they disappear. With good fixation, you should see a strong greenish colour whenever the violet patch has disappeared.
When you are fixating well, after a few cycles you will actually see a rotating green spot! If your gaze is really steady, the magenta patches will disappear, leaving only a rotating green spot!
The temporal presentation enhances the well-known afterimage in complementary colour.
Jeremy Hinton writes: “The illusion illustrates Troxler fading, complementary colours, negative after-effects, and is capable of showing colours outside the display gamut.”
Source: Jeremy L Hinton (2005-05-22)
*Note: There’s a yin-yang thing going on here with the green and magenta. The green your brain is seeing happens to be the complementary of the magenta, which puts it on the opposite end of the color wheel. For more info on what this means, check out the Color Theory post.