What to see
There are two disks, pulsating between dark and light. Are they doing this simultaneously? If you contine watching, you will notice that this depends on the background. If it is light, the two disks are in synchrony. However, when one background is dark and the other light, they pulsate with opposite phase.
What to do
Deselect “Autorun” and move the background slider fully up. Now, using the rightmost slider, find the position where the two disks pulsate simultaneously (in phase). For me that's at the -180° position. Now move the background slider fully down, revealing that the disks are not pulsating in phase at al.
What happens here is that the brightness of a patch is influenced by its surround luminance. “Luminance” refers to physical properties (how many photos per second are emitted at a given location), “brightness” refers to the subjective assessment. The modification of brightness by surround is a “simultaneous contrast” phenomenon. Arthur Shapiro and colleagues developed this demonstration, which is particularly effective in bringing out this phenomenon by giving the impression that the two disks pulsate in antiphase (which is caused by the differerence in surround luminance). This and more is discussed in their paper (below).
Shapiro AG, D’Antona AD, Charles JP, Belano LA, Smith JB, Shear-Heyman M (2004) Induced contrast asynchronies. J Vis 4:459–468