It features a black circle, which actually stays the same size, but appears to change when surrounded by a set of circles that grow and shrink over time.
The society notes: “Interestingly, this effect is relatively weak when looking directly at a stationary central circle. But if you look away from the central circle or move your eyes, or if the entire stimulus move across the screen, then the illusory effect is surprisingly strong – at least twice as large as the classic, static Ebbinghaus illusion.”