It's water weight loss. Potassium competes with sodium. You take enough potassium and it becomes a diuretic.
It never stops amazing me when weight manipulation curious people just ignore the entire history of bodybuilding and weight class sports. A standard American diet, especially if you ever eat out, is very high in sodium, and a high sodium:potassium ratio is one of the chief reasons people retain excess water. Either reducing sodium or increasing potassium is a time-honored way to quickly lose water weight that people in sports have been practicing for decades.