Velo-
I do not think that really works- I think it just makes you more efficient at breathing through your nose, that's all.
If it really was that effective, I think we'd see lots of athletes running around with clothespins on their noses while doing their chores and while training too.
They used to talk about that nonsense with the spinning program, and they talk about it a little in yoga, but in yoga, they usually talk about nose breathing as a calming technique to help bring more oxygen into the lungs- that's about all I remember from that. The spinning folks try to do that too, but it's definitely not used to try and deprive people of oxygen so they can learn to work hard without it or something like that.
Koffee