A rhythmic whole-body motion that briefly produces more drive than the wind is actually giving you. Below roughly 18 knots it often decides whether you get planing at all.
Pumping is the cheapest extra wind there is — and the most commonly botched. The classic error is pulling with the arms alone: it torches your forearms in minutes and barely moves you. The impulse comes from legs and torso, with the arms only passing it on. Second error: pumping too high on the wind. Bear away slightly first, so every burst goes into forward drive instead of leaking sideways. And pump deliberately rather than constantly — just below your planing threshold, or when a gust is dying. Your wake tells you whether it worked: the broad foamy trail narrows into a smooth, thin line. Two things people miss: in racing, pumping isn't free everywhere — unlimited in slalom, but only for the first 30 seconds after the start in Formula and course racing. And on a windfoil, pumping means something else entirely: a circular motion of the whole rig that lifts the foil.