How do you get bigger, faster, stronger lungs? According to Bas Rutten you do it by making you lungs work harder using the O2 trainer. You put the device in your mouth and breathe through it. The size of the hole on one side of the device, which can be adjusted according to how strong your lungs are, determines the amount of resistance it takes to breathe in. Unlike the Elevation Training Mask, there's no resistance for breathing out, which as it turns out is a good thing because you'd want to exhale all the stale air in your lungs before inhaling fresh air. It's also cheaper and easier to carry around. However, if working out with a mouthpiece is not your thing you should go for the Elevation Training Mask instead.