Yeah, it seems counter to what I've learned too. The idea of keeping oxygen in your lungs doesn't hold water (ha ha) because your body doesn't sense oxygen levels. It senses CO2 levels. If you are in a room with no oxygen you wouldn't notice so long as you can get rid of the CO2. That's why people get carbon monoxide poisoning, or suffocate without knowing it in a room full of gas nitrogen. Breathing out continuously lets your body keep getting rid of CO2.