You put one hand over the end of the m/p and suck at the reed end. The reed will adhere to the tip and rails of the m/p before "popping" off. It's a nice way to straighten a warped reed but i can't believe it's reliable for telling you if a m/p needs refacing etc. Reeds are flexible - that's the whole point of them, so it would adhere even if there were some irregularities, i think. If the m/p was so warped that you couldn't do a "pop test" then i reckon you'd be able to see it with the naked eye or with a straight edge held against the rails/table.