Yes, we all breathe already, and all assume we are doing a fine job of it. However, I went over 30 years before I learned how to do it correctly. I had heard the insult "mouth breather" before, but didn�t understand... I have always been (honestly) among the smartest people I�ve known, and I used my mouth, you have to in order to speak, and martial arts teach to breathe in through the nose, out through the mouth.
WRONG!!! Bad mouth-breathing tailless monkey!
Seriously, though, we found more than a few places, all just a bit off IMHO, because they practice the end result, how you end up breathing all of the time, as an occasional (or at most daily) meditative form. We have since found other sources that detailed a way to work up to using full lung capacity all of the time. Some were quite decent, but a bit excessive in a number of things, including bathroom rituals, which I will spare you. You�re welcome.
I was able to formulate a method out of the combinations of all these things, and am now very aware of how much better overall I am. I don�t think that they were far off about most "Westerners" only running at 40-60% capacity because of improper breathing. I�m not sure, but it seems like I am faster and stronger as well, and I am definitely more in control of my emotions, and feeling less like life is the verge of a crisis all the time.
Posture is actually an important factor. Slouching partially compresses the lungs and does not allow use of a good portion of them. It also makes it much more difficult to expand the ribcage even slightly. It affects other aspects of life as well, from the alignment of the spine to how others perceive and listen/respond to a person.
Our emotions affect our breathing, and the reverse is also true. When we are in a panic, or in crisis, we automatically breathe more rapidly & shallowly. Therefore, by breathing more shallowly than our lungs are designed for, we are subjecting ourselves to such a physiological state perpetually.
I suspect that the gas exchange is probably a huge part of it. When you are fully inflating and deflating your lungs at a consistent rate, the body may be more able to extract all of the oxygen from the air, and get all of the wastes out. It is possible that there are more waste products than just the carbon dioxide, just as for so long we were completely unaware of the chemical releases involved with crying.
I�ve read a couple of variations on how to work up lung capacity, most commonly it seems they segregate the diaphragm muscles into two sections, top and bottom. I have also seen that stated as "breathing with the stomach" and "breathing with the chest". Their idea is to first inflate one way, as much as you can, then the other to add a bit more, alternating until no more can fit, then breathing out at the same rate without pause in between. Some said that outward should follow the same pattern in reverse, some apparently assumed that out is out.
I see it as three ways to inflate the lungs. There are the two aforementioned, the stomach and chest, there is also what I picture as "the barrel" of the chest, using both together evenly instead of alternating. For normal every-moment breathing, that is a lot to keep track of, and certainly not to be attempted while trying to do anything else, especially something as dangerous as driving! So, thus, I use the "barrel" regularly, and have even trained my body to breathe that way when unconsciously controlling it for me.
The exercise can still be useful, perhaps daily or as often as you like, to build yourself up. Although we are constantly using them, those parts could also use regular stretching and exercising. I do so regularly, as I sit at the computer, and when I am walking, I end up conscious of my breathing, counting the strides and forcing just a little bit extra each time, so it is almost uncomfortable but I am sure that my lungs are as full as they can get.
This is a lot to cram into an article, so I will write a (hopefully short,) more comprehensive book on it soon. That will also include more of a bibliography; here I didn�t use any particular other works much at all, and listing those I cross referenced would be lengthier than the article.