Wow, that’s a mouthful, so how about a video to make it a bit clearer?
Basically, it’s a type of optical illusion, making a static image appear to animate, by taking a card with slits in it, and dragging it over the image. Of course, the image itself isn’t normal looking by any means – at best, it looks like a mess of blobs and lines – which is where the whole “optical illusion” comes in.
For example, here’s a colorful one that I made.
So, what’s the mess about? Well, it might be a little easier to understand, by looking at what the slits in the card does. Namely, each slit shows a portion of a single animation frame, and the space between slits, is equal to the width of the slit, times the number of frames in the animation, minus one. That “one” being the width of the slit.
Since that space between slits blocks out large portions of any given frame, it’s essentially “dead” space, that can be used for the other frames in the animation. Each frame has the dead space removed, and then is shifted over one unit to the right from the frame before it. Finally all the frames are merged together to form one image.
This is why moving the card from side to side, makes it appear that it’s animating – as each unit the card moves over, the slits display a different frame. Below is that same image as above, this time with an appropriate “card” over it. Drag the card right or left, to see the animation.