It's the same test the divers take. They take you "down" to a certain depth (pressure) and back up. The idea is to make sure that you can equalise pressure differences.
It doesn't really mean a whole lot...when a wave washes over the snort mast, or if you're doing "tactical" snorting, or if the Sea Things/Canadian Aviator's ilk are about, or if the Docinexperienced planesmen are on the OMC, you'll do a lot of "manual" pressure equalisation....or you'll bleed from your ears. Either way, no big deal.