The Mahayana Zen I practice has a saying, "Stay with your breath." All that means is pay attention to your breathing. Breath control can lead to some pretty amazing things. Probably the most impressive that I know of is people with deep meditation skills slowing their heart rate down to the point that they don't bleed out from serious trauma.
Breath control is also an excellent way to control your mood. I use it to calm down when I get angry, and to slow my mind or mouth down when I get too spooled up.
Regarding martial arts, many, many people hold their breath while sparring. This is a very common mistake that can plague even veteran practitioners. Heck, even though I've been working on even breathing on the mat for at least five years, I still occasionally hold my breath when I'm in a bad spot. If you don't know already, holding your breath is almost never a good idea when sparring. Flatulence and framing your diaphragm momentarily are the only things I can think of that would be exceptions.
I have also been taught to breath in through the nose and out through the mouth.
What you're describing is what occurs during zazen (seated Zen/Chan meditation) which is what I practice and jnp as well. I know there are differences between Taoist and Buddhist arts with regards to breathing, but this is at least how I've been taught on the so-called "external" side of things.
Originally Posted by Diesel_tke
Zazen is a big part of why I am in a much better place mentally than two years ago. I'm still not consistent enough, but zazen is the only thing that prevented a nervous breakdown on my part a few years back. When the student is ready....the master appears and all that.
Originally Posted by jnp
If you want to talk benefits....in my wife's own words, practicing zazen has made me "virile". I think you know what that means.