What to see
On the right there are 2 blue dots which continuosly change position. Decide for yourself: are they moving (hopping) horizontally, vertically, or (counter-) clockwise?
What to do
If you see them moving horizontally, cover the left two positions with your hand. Now they are moving vertically, and may keep on moving vertically if you remove you hand.
If you see them moving vertically, cover the bottom two positions with your hand and observe.
Another way to switch the seeming motion direction is via the buttons “aspct↑” (short for “increase aspect”) or “aspct↓”. Once you get the hang of it, you may be able switch it mentally, especially if you find an aspect ratio (using the slider ) where the two percepts are equally likely.
You even may be able to perceive a rotation, clockwise or counter-clockwise (like a propeller).
Question: What is the “real motion” of the discs? Answer: None; there are actually 4 discs staying put, just their visibility is switched. The grouping to horizontal, vertical or rotatory motion occurs in our brain.
This belongs to the class of multistable figures and countless research papers have been written on it, starting with von Schiller 1933. The figure is also known as “motion quartet” and, unfortunately, recently a new name “ambiguous quartet” was ascribed (Carter et al. 2008, interestingly in a tactile version).
von Schiller P (1933) Stoboskopische Alternativbewegungen [Stroboscopic alternative motion]. Psychologische Forschung 17:179–214
Demo of “clustered bistable quartets” (and more) on Peter Schiller's site
Ramachandran VS, Anstis SM (1986) The perception of apparent motion. Scientific American 254:102–109