I think this addresses the painfully inadequate instruction that goes with certification now. Thinking back, I don`t think I knew anything about port plugs either but should have. Hell, I thought it was almost magic that happened inside a second stage that enabled me to breathe under water. I have seen a lot of people that were clueless to a lot of things they shouldn`t be because an instructor just wanted to get paid. Just glad the OP thought to ask. Too many people don`t and have a bad experience.