My understanding from the book was that CO2 is an essential part of the mechanism that transfers oxygen to your muscles. If you breathe too much you have too little CO2, your muscles can't get enough oxygen and they don't work efficiently. I've no idea if this is true, but since I read the book I breathe less when exercising and it seems to help. Edited to add: I'd forgotten, when I first started breathing less during exercise it made a huge difference, like coming down from altitude. I recommend it.