Yep, its all to do with controlling your oxygen intake and improving performance.
More oxygen = more energy
It was part of an article on 10 ways to improve your running.
OK, I'm no triathlete, nor even a good runner, but if I can learn some techniques that will better my performance and improve my fitness then I'm up for it.
Controlled, rhythmic, deep breathing (using your belly) utilises more of your lung capacity with a greater output of energy. Breathing in through your nose and out through your mouth regulates rhythmn.
Apparently if you cant breath regularly in through your nose then you are overdoing it, so its says anyway. So by definition, I am overdoing it all the time.