Copyright George Mather, 2003
George Mather, the creator of this piece, had this to say about this illusion.
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:Go to his site for a more in dept analysis of this and other illusions.
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).