IFLI
Two FLI pictures shown on alternate frames and offset by one pixel, so the eye blends colours and resolution the C64 doesn't actually have.
IFLI stands for Interlaced FLI and builds on FLI mode. The computer flips between two complete images 50 times a second on PAL. One of them is shifted a pixel sideways, which gives the fat pixels of multicolor mode an apparent horizontal resolution of 320, and where two different colours land on the same spot the viewer sees a mix of the two.
The price is flicker — obvious on a sharp modern display, softened by the phosphor persistence of an old CRT television — plus twice the memory of an ordinary FLI picture. That is why IFLI lives mostly in still images for slideshows and demos; a game in this mode would have had neither memory nor CPU time to spare.