Video feedback
Demo effect where the screen image is fed back into itself every frame, producing endless tunnels and lagging trails.
The trick imitates what happens when you point a video camera at its own monitor: the image contains a smaller copy of itself, which contains a smaller one still, on to infinity. In a demo it is done in software — the previous frame is copied back, slightly scaled, rotated or offset, before the next layer is drawn on top. The result is swirling tunnels and colour trails that seem to pull into the screen.
The effect was cheap and striking, especially on the Amiga where the Copper and blitter could move whole buffers quickly. It is a cousin of the tunnel effect, but arises from the feedback itself rather than from a precomputed map.