Chooses between two pictures by comparing a map against a cutoff. Above the line, you see one; below it, the other. Either input can be the map.
The comparison can run on brightness, or on each colour channel separately.
Per-channel is the more interesting mode. With its own cutoff for red, green and blue, the choice gets made three times over, and a single pixel can end up taking its red from one picture and its blue from the other. What you get is colour separation rather than a clean cut, with fringes of pure colour where the three decisions disagree.
range is the width of the fade either side of the cutoff, so at zero it is a hard-edged stencil. Stepping the map into discrete levels first turns smooth gradients into banded regions before any of this happens.