Warning: this is a subtle effect.
What to observe
There are 4 panels: 2 bar sequences a the top, and 2 motorcycle sequences at the bottom. Ask yourself in which you perceive (nearly) smooth motion.
On the left, there is a fairly smooth motion percept for the motorcycle. The right one obviously only switches between 2 positions.
Let’s describe the motorcycle ride, the principles apply also to the bars. In the left row, there are only 2 different positions of the motor cycle, these form the first 2 frames. The remaining 2 frames (there are only 4) are comprised of image negatives of the first 2 frames. The left bars show a never ending, if strange rightwards motion. The right bars show a (somewhat jumpy) rightwards motion (phi phenomenon), with an obvious jump back from the last to the first bar.
Anstis S (1970) Phi movement as a subtraction process. Vision Res 10:1411–1430
Anstis SM, Rogers BJ (1975) Illusory reversals of visual depth and movement during changes in contrast. Vision Res 15:957–961
Anstis SM, Rogers BJ (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception 15:627–640
George Mather (from there I extracted the motorcycle and simplified it further. Thanks!)