This animation involves a repeating cycle of two different pattern frames (hence 'two-stroke'), with a blank grey frame after the second frame. The motorcycle repeatedly moves forward (pattern frames 1-2) and then backward (frames 2-1), but the blank frame intervenes between each backward shift from frame 2 to frame 1. 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 back-and-forth motion as the two pattern frames alternate. But the effect of the blank frame is to reverse the direction of any movement perceived by the transition from the pattern in the frame before the blank to the pattern in the frame after the blank. So instead of appearing to move backwards from frames 2-1, the motorcycle still appears to move forward. Consequently the impression is of continuous forward motion. This kind of display was first described by Mather (2006).
Two-stroke apparent motion can be used as a tool to study how visual movement is processed in the brain. Many previous studies have discovered neurons in the brain the respond only when moving patterns are present in the image. A mathematical model of the neural circuit that feeds these neurons (described here) can explain two-stroke apparent motion. For a detailed account of the explanation, setting it in the context of other motion illusions, see Mather (2017). More technical accounts can be found in the journal articles.
See:
Mather, G. (2017). Two-stroke apparent motion. In: A. Shapiro & D. Todorovic (Eds.) Oxford Compendium of Visual Illusions. OUP, Oxford. (pp.531-535) PDF
Challinor, K., & Mather, G. (2010). A motion-energy model predicts the direction discrimination and MAE duration of two-stroke apparent motion at high and low retinal illuminance. Vision Research, 50, 1109-16. PDF
Mather, G. (2006). Two-stroke: A new illusion of visual motion based on the time course of neural responses in the human visual system. Vision Research, 46, 2015– 2018. PDF
Shioiri, S., & Cavanagh, P. (1990). ISI produces reverse apparent motion. Vision Research, 30, 757–768.