The trick: the field is computed once. Four overlapping sine terms per pixel, summed and folded into a byte 0–255. That byte is a palette index, not a color.

Every frame after that, nothing in the image changes. Only the 256-entry lookup table rotates by one. Your eye insists it sees flowing liquid; the pixels have not moved since the first frame.

Tick the checkbox to recompute the field each frame instead. It looks similar and costs roughly a hundred times more. On a 486 that was the whole difference between a demo that ran and one that didn't.