This is a slit scan movie of ducks swimming.

Imagine video footage is a box. With each frame stacked behind the previous frame. The front side of the box is how we traditionally look at a video. Think of the front plane as the XY plane. The sides of the box, the planes down the XZ or YZ are slices that show how those pixels change over the duration of the video.

I work in 3D a lot so thinking of a 2D object (a video frame) as a 3D cross-section kinda makes sense. It's a little like taking reality and viewing it from the side.