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DeeperBlue.com: Technical Freediving: Are Breathhold Divers Ready To Mix It Up?.
Fascinating article, I like respiratory physiology anyway, but some of this is a bit out there with interesting risks. Blowing off CO2 with hyperventilation and flushing nitrogen to increase your O2 reserve gets really interesting with respiration not ventilation. Similar in the stuff with "Packing" your lungs with a single breath at 6m to increase capacity. Not a free diver by any means, but would be curious to play with some of this.