The illusion was invented by Jeremy Hinton some time before 2005. He stumbled across the configuration while devising stimuli for visual motion experiments. In one version of a program to move a disc around a central point, he mistakenly neglected to erase the preceding disc, which created the appearance of a moving gap. On noticing the moving green-disc afterimage, he adjusted foreground and background colours, number of discs, and timing to optimise the effect.
Stare at the central cross.
You will see a greenish colour disc wherever the pink disc has disappeared.
Eventually, if you stare long enough, the pink discs will also disappear and you'll just see only a moving green disc.