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.
An improved version of this won George Mather the second place in the Illusion Contest 2005.
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!)