You start by (sprint) swimming 50m freecrawl. Take a breath every 2 strokes. The next 50m you take a breath every 4 strokes and then continue to pyramid up and then back down. On a good day I can get to 10 or 12, but it's freaking tuff. Hypoxic drills build up your lungs, help endurance/stamina, breathing control and help to develop a mental toughness. This can even be done during a weight training session such as squats/ deads etc. but you would probably see people such as crossfit, Seals, recon, sf guys doing things like that. It's that strength, enduance, stamina deal we all strive for. I personally only do it with swimming and can attest to how well it has helped me to become a stronger swimmer.