When set to the open position, the bung acts as an airlock and allows the release of CO2 while maintaining a seal against oxygen coming in. This makes the bung ideal for alcoholic or malolactic fermentation in barrels or tanks. When fermentation is over and you no longer want the bung to "breathe" out, you just push the center cap into the bung to create a firm seal and the bung becomes a solid stopper.
