Back in my early Amiga days (A500), I remember coding simple demos that would use the dual-playfield mode and scroll them around (at a sweet 60FPS). It was pretty trivial to do with languages like Blitz Basic and AMOS. My knowledge back then was pretty limited as to what the Amiga could really do.
Later on in life, I learn that games like Agony used tricks like putting a static 1-bit bit plane and simply altering the background color of that bit plane with a copper list. Then, it would use the dual-playfield as normal with the exception that the background would now only have 2 planes instead of 3. This effect gives the illusion of three layers of scrolling with little CPU overhead.
This is amazing!
So my question is, how were the individual bit planes controlled like that? I originally thought that the grouped planes had to "stick together" when they move around. But I was wrong.