Initially, you see a wireframe representing a rotating head. Question: in which direction does it rotate? Most people (including myself) do not see a full rotation, but rather a ≈180° left-right oscillation (most of the time). But what does the head really do?
Now use the pop-up menue which initially reads
Wireframe and select
Silhouette. This poses a more difficult problem: Does the head now rotate always in one direction, or does it oscillate like with the wireframe?
Finally, the pop-up menue offers
Full3D which leaves no doubt about the rotation direction.
There are more controls for your experimentation.
With the silhouette, it is well known that the perceived rotation direction is arbitrary (you can disambiguate by “unhiding” the eyes); this is the topic here. The information is equally compatible with leftward, rightward or oscillatory motion, no need to argue. With full 3D, there is no ambiguity at all.
So what happens with the wireframe? Why does this always oscillate? My hypothesis is that here the “hollow faces” illusion comes into play. When the face looks away from us, this looks like a hollow face mask, where immediately our face detectors activate and flip it (hollow faces are rare in reality), thus overriding the history (and the weak 3D hints, the eyes are a wee bit smaller when behind). Consequently, the “real” full rotation is not perceived.
I thank Swarnava D. for pointing me to a video which, among other things, showed the effect of a wireframe face. I will not link to that video (Bright Side), because all explanations there are either totally wrong or semantically tautological. But the effect is nice and prompted me to program the above demonstration, so thank you!
The head model is from Free3D, but in spite of the alluring name of this site, it was not free ;)