This animation involves a repeating cycle of four different pattern frames (hence 'four-stroke'). The motorcycle repeatedly moves forward (frames 1-2 and 3-4) and then backward (frames 2-3 and 4-1), but each backward shift is accompanied by a reversal in contrast: All dark points in the image become light and vicer-versa. Use the movie's playback control to examine individual frames (press the Pause button and then use your keyboard's arrow keys to advance through frames).
This sequence of displacements would normally lead to the perception of to-and-fro motion. But the effect of contrast reversal is to reverse the direction of perceived motion, so instead of appearing to move backwards from frames 2-3 and 4-1, 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).
The illusion seen in four-stroke apparent motion can be explained by responses in specialised neurons in the visual system which respond selectively to motion direction and stimulus contrast. For a detailed account of the explanation for this effect and for a related effect (two-stroke apparent motion), see Mather (2013).
See:
Anstis, S.M. (1970). Phi movement as a subtraction process. Vision Research, 10, 1411-1430
Anstis, S. M. & Rogers, B. J. (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception, 15, 627-640.
Mather, G. (2013). Two-stroke apparent motion. In: A. Shapiro & D. Todorovic (Eds.) Oxford Compendium of Visual Illusions. OUP, Oxford. PDF