This animation involves a repeating cycle of four different frames (hence "four-stroke"). The motorcycle moves forward and then backward again (the scenery moves backward and then forward again), but the backward step is accompanied by a reversal in contrast:
The effect of contrast reversal is to reverse the direction of perceived motion, so instead of appearing to move backwards between frames 2 and 3, and between frames 4 and 1, the motorcycle still appears to move forward. Consequently, the impression is of continuous forward motion. This kind of display was first described by Anstis & Rogers (1986), and is closely related to 'quadrature motion' displays (Carney & Shadlen, 1993).
How can we explain four-stroke motion? The explanation becomes clear when we think in terms of xyt space. A simple four-stroke display would look like this:
A dark bar in frame 1 shifts rightward (2), then back to the left with a contrast reversal (3), then rightward again (4), and so on.
The pattern of alternation in contrast over space and over time neatly
aligns with the receptive field of a low-level motion detector tuned to
signal rightward motion (dotted lines), but offers no consistent stimulus
to leftward detectors. The result is consistent apparent motion in the rightward
direction.