Mid-frame palette change
Writing new colours to the palette while the beam is still drawing, so the screen shows more colours than the palette nominally holds.
The palette governs the whole screen — unless you change it partway down the frame. By swapping palette registers on a chosen scanline, timed with a raster interrupt, the Copper or a sprite 0 hit, the top and bottom halves could show different colours. C64 demos stacked dozens of such changes into smooth raster bars; the Amiga's Copper made sky gradients almost free. Unlike colour cycling, which rotates the palette in time, this divides it in space. The payoff was greatest on machines with a small palette and large ambition.