Copper

The Copper was a coprocessor of the Amiga's chipset that could be used to change video settings while the cpu was busy doing more important stuff.

It was often used to change the color registers for different rows of an image, allowing the Amiga to show more colors on screen at once than it was actually capable.
Being independent of the cpu made it possible to have these effects practically for free (i.e. without using up precious cpu time)