This, and the ergonomics of a lighter pen, is the reason I prefer the capped Pilot Precise BeGreen pen, which accepts the same V5 cartridges. I'm surprised how relatively rare these have stayed when the retractable V5s are everywhere.
From my understanding, the deep breathing doesn't particularly raise O2 (which is already 97-99%) but purges CO2, which then lets you hold your breath (empty lungs) for 2-3 minutes. In this period, my O2 (on my finger) drops to 85% before I feel a need to breathe in. It is important to 'replenish' the CO2 between deep breathing, otherwise I get very loopy.