Above you can see a number of shapes, stacked on each other. Perhaps you even see a tower (a smoke stack)?
If you already see a tower, fine. All others click “Auto rotate” or simply press the space bar (touch interface: tap the tower). Now it turns into a rotating tower! A leaning tower, on to which you look from atop, and it wobbles around. This is “Duncan’s Leaning Tower”, which he evolved from the old Benussi & Musatti effects, which don’t really work for me.
From the pop-up menu you can also choose “Duncan’s Racetrack”. What you might observe here is (after pressing the space bar) that the racetrack seems to deform rather than simply rotate. If you click on “Dashed” it becomes obvious that it’s just rotating.
It has been observed repeatedly that slow rotation of certain shapes can give rise to a three-dimensional percept (e.g. Musatti, 1924, also my comment here). Independently, Duncan and Duchamp developed this into stronger effects. The title I choose above is the one Duncan (1975) uses to describe his art in his Leonardo article.
Ian Howard (2012) gives a full quantitative description of kinetic depth effects in section 28.5 of his book; Vezzani et al. present a comprehensive review.
Musatti CL (1924) Sui fenomeni stereocinetici. Archivio Italiano di Psicologia 3:105–120
Duncan F (1975) Kinetic art: On my psychokinematic objects. Leonardo 8:97–101
Vezzani S, Kramer P, Bressan P(2015) Stereokinetic effect, kinetic depth effect, and structure from motion. In: Wagemans J (ed) Oxford Handbook of Perceptual Organization
Howard HP (2012) Perceiving in Depth, Volume 3: Other Mechanisms of Depth