Oxballs Drain-O Flow-Thru Buttplug Bk SmDRAIN O flow thru plugs are designed for max stretch no matter the size you choose. Each plug is designed with special features that help keep DRAIN O in place. DRAIN O has a pencil thick airhole vent through the center of the plug? the design helps ? equalize? the pressure inside and out. That feeling you get when you want a plug to stay put but your muscles want to expel it is partly built up pressure behind the plug? this feature helps with that.