What to see
Above a black grating with narrow slits moves to and fro. Partially covered by it, there are blue balls eaten by the pac-man at the left, at least while covered with the grating. When the blue parts are not covered, they look rather disheveled.
What to do
Grab the black grating with your mouse and move it about (that switches off “Autorun”). Now you can make the balls move by yourself.
Shift the grating up a little, so that it only covers the top half of the blue parts, then move it right-left. It's rather strange how only the covered parts seem to move.
In the “Autorun” mode it's also interesting to observe the effects of changing the number of animation steps (more below) and the with of the gaps in the grating.
I hope all of you have played with a flip book, possibly even drawn a simple one yourself? If not, I absolutely suggest you do so, great fun! It's also a nice creative project for school. In a flip book, you draw a simple scene, where from page to page something moves; then you rapidly flip through the pages. The rest is done by our brain: from just a few phases of motion it creates a seeming continuous motion percept. Technically this is known as the “φ-phenomenon”.
Here, the same principle applies: several steps of a scene with motion are drawn, but the various steps are drawn only in narrow slits, slightly offset from step to step. The black grating on top has slits just as wide as the slits used for drawing, and thus reveals only a single step at a time. Moving the grating reveals one motion step after another, and our brain stitches the partially covered blue shapes together (known as “gestalt completion”, here is an non-motion example for that).
While “scanimation” was historically referring to some special movie animation tricks, this has now been adopted for the technique demoed above. There are wonderful books available, where a plastic transparency with a grid on it is used to cover the page and reveals fanciful scenes when moved. The technique is closely related to Moirées, which I demonstrate here.