The importance here is t he difference between resistance training and just breathing hard. All Cardiovascular activities make you breath harder and challenge the body to use O2 more effciently.

The Breathing resistance idea is that you are doing strength training for the breathing muscles. resistance training effects the muscles on a celluar level in many different ways. One effect is that the muscles get stronger, but the cellular metabolism and energy use changes also. If your respiratory muscles are stronger you will be able to move more air more efficeintly. In the abstract form the original study it indicated a change in tidal volume or the amount of total air moved in a breath. If you can move more air easier you will be able to work harder with less effort. a 33% increase is very significant. A 10% increase would be noticeable. Athletes at the top of their gain usually are looking for 3-4% advantge over an opponent.

I understand the Apnea thing completely. I used to use a CPAP machine at 12mm/hg. nothing like sleeping with a constant wind in your face. I think the practice i got at night from not breathing helped with my long breatholds however.