I saw this link last fall and after reading through their website have been playing with the technique ...usually on climbs. It basically strikes me as yoga breathing with some twist.
http://www.breathplay.com/

I like it ... I think it slows your breathing rythem down a little when you are at your max and as someone else mentioned that gets more O2 in your blood as your lungs have more time to extract the O2. I also think there is a mental aspect there that takes your mind off your suffering and makes you concentrate on something else. Whatever the reason I know I am less winded at the top of steep high exertion climbs when I do it.