Some powders due to their fine nature and the viscosity of water, are very resitant to permitting water to soak in. Just like the way water beads and rolls off a newly waxed car finish, but soaks an old unwaxed finish. Black powder is hydroscopic, attracts and soaks in moisture, even from the air. Some of these powder act in two ways, they cause water to bead and roll off, and some are even more hydroscopic than the powder, meaning they soak in the moisture faster than the black powder. (Kind of like those little dessicant packs in bags of store bought beef jerky.) It keeps the pan powder dry, to an extent. Probably still won't go off under water.