I assume that some people are aware of "Reverse Breathing." The blog that Mike mentions, I use "Reverse Breathing" more as an awareness exercise than for the typical purposes that "Reverse Breathing" is used for. Most people expand their upper chest when breathing in, while relaxing their diaphragm. My use of "Reverse Breathing" is for paradoxical intent. Having students connect their purposeful use of their diaphragm with how most people typically breathe, helps them to transition toward allowing their diaphragms to expand down into their groin area when breathing in.
Reverse breathing is a valuable tool for a number of things. Of course there is also "Ki Breathing." What other breathing methods to people employ and why?
Marc, let me make an observation with good intention: if I was a relative newby to Aikido or any other art, I would read your discussion and maybe get the idea that there are two different kinds of breathing. However, in addition to a not-really-clear definition of reverse-breath, there's no real clear reason what the usage differences are between normal/natural/post-birth breathing and reverse/pre-birth breathing.
And what is the point of "allowing their diaphragms to expand down into their groin area when breathing in"? Not trying to be picky here, but it's my habit to always read with an idea toward "what is the usable information here?".
My original comment was simply that in any type of breathing, on the inhale the diaphragm must come downward or air couldn't be pulled into the lungs. This is true for both 'reverse breathing' and for 'natural breathing'. "Ki Breathing" must be one of those two types. I dunno... it depends on what "Ki Breathing" is and that hasn't been defined; only labeled.
Tohei and his students have indicated that there are some types of breathing exercises that he hasn't made publicly available. My bet, based on knowing some breathing exercises from a number of Asian systems, is that the full breathing complement involves some instances of reverse breathing also.
Given that reverse-breathing is found pretty much everywhere in Asian martial-arts (i.e., it's no big secret), what do you think it's used for functionally, Marc?