"hypoxic" training?

i always took hypoxic training to mean training at altitude, so you deal with lower oxygen levels. Lower oxygen levels in the air means your body needs more hemoglobin to absorb it efficiently. Build up a ton of hemoglobin and go down to a lower altitude and you have lance armstrong lungs.

Not sure of the effect of CO2 on this is, but I imagine any way of training where you are limiting the amount of oxygen you get would do this

There's no evidence to suggest that doing "hypoxic sets" provides any performance benefits in swimming, and I doubt that you would find any benefit in holding your breath while cycling or running.

The only advantage of doing hypoxic sets would be to increase your comfort level in the water. This could pay off in a race where you're getting beat up by other swimmers, or you're in choppy water and taking a face full of water.

As for the second post that discusses hypoxic in the sense of being in an environment with less oxygen (higher altitude), there is a benefit in long term exposure through the natural build up of red blood cells to handle the thinner air. A higher red blood cell count can certainly be advantageous to an endurance athlete.

scott