What to see
A ring of large blueish disks and a smalle orange disk, and everything moves. The blue disks change size, does the orange disk change size too? If you're a little like me, you see the the orange disk as smaller when it's in the middle, and growing larger when moving to the bottom right. The effect increases when you fixate the blue dot on the top left blue disk.
What to do
Activate the “Guides” checkbox. A pair of parallel (!) lines appears to indicate that the orange disk has constant diameter throughout. However, the illusion is so strong that the lines seem to bulge!
Activate the “Compensation” checkbox. When the slider is at the bottom, nothing changes. Move the slider up, and the orange disk diameter is now dynamically changed. Try to find a slider setting where the orange disks is close to constant in size throughout (for me that's near the slider's centre). Now switch on the “Guide” lines again, and you can estimate the size of the illusion.
Obviously, the context – here the surrounding smaller or larger disks – affects our size judgement.
This is a dynamic version of the “Ebbinghaus Illusion” (see there for more). The dynamic variant, created by Christopher D. Blair, Gideon P. Caplovitz and Ryan E.B. Mruczek, enhances illusion strength and was a deserved winner at the 2014 “Visual Illusion Contest”.