Dual playfield
The Amiga's hardware trick of stacking two independently scrolling image layers — parallax without the CPU lifting a finger.
The mode splits the Amiga's bitplanes into two separate playfields that can scroll independently, one in front of the other. Even bitplanes form one field, odd ones the other; colour 0 in the front field is transparent and lets the back one show through. The result is depth and parallax drawn by the custom chips while the 68000 does other work.
The technique was the backbone of smoothly scrolling Amiga games from the late 1980s — Shadow of the Beast layered field upon field until the background moved at several speeds at once. The cost was fewer colours per field, so demo coders often switched modes mid-screen using copper lists.