Very interesting article, thank you! So, basically, if CP really does not correlate with CO2 levels, most of what I read about the Buteyko method was total BS.
But I understand what you mean by saying CP is just a measurement tool. Even so, since increased CP & MP would constitute improvement, I think it is fair to classify increased breath holding time as a goal of the training. That's s matter of definition and is not important, of course.
You're right, but still, CP seems to be the more intelligent approach because it can be done by even severly ill patients and it's less depending on your mental state. If you psych yourself up you will always be able to hold your breath much longer.
As to Rakhimov's claims. Here are some of them on his YouTube channel:
How to Get Rid of a Stuffy Nose - Clear in 40 seconds
Constipation Remedy: Cure in 1 Min (Breathing Exercise)
How to Get Rid of Anxiety Naturally in 1 Min: Easy Breathing Exercise
I would rather call that symptoms than diseases, but that's probably a matter of definition. I tried the first one several times and it actually works great, but it doesn't last that long. You're supposed to repeat it until your nose stays clear. According to Rakhimov it's because the CO2 needs to accumulate until it has reached a certain level.
Personally, I think that something else is going on: When you're doing these breath holds you experience stress (feeling of suffocating). When this stress reaches a certain level your sympathetic nervous system gets ramped up in order to protect you. The fight or flight reaction takes place and I think that clear airways are a good thing when you need to fight or run for your life. So maybe pain/suffering/stress could be the reason behind this remedy. Prolongued progressive breathing exercises on a regular basis could mean so much stress that your sympathetic nervous system gets turned on and stays more active throughout the day resulting in clear airways, more energy, less hunger, less need to sleep and increaeed focus. All of this is listed as a direct effect of breathing training, hence increased pulmonary CO2 and cellular O2 levels. But I think it kind of sounds like a description of the fight or flight reaction. Maybe this whole breathing training is not about CO2 after all, but a change in how your nervous system is operating in general. It could be explained by the priciple of hormesis: you impose a constant but mild stress on your body by breathing less and get several health benefits in return, because your body reacts by several non-specific adaptions. That's just a theory of mine, I don't know if it makes sense. I'm a bit confused right now, you broke my understanding of breathing physiology with that article, thanks for that