Breathing is something we all do and have done so long that we may not notice it. I started to really pay attention to it when I began to meditate, and now I'm trying to pay attention to it more often. Here are a few things I saw in myself.
There are a lot of times during the day when I'm not breathing fully - short, shallow breaths, or lightly holding my breath. Yesterday I noticed this when I was driving and as a result of it I had felt really, really tense. Just giving a few moments to get a deeper breath going gave me a sense of calm. I do this a lot. I'm not sure why, yet.
When I first started running, I noticed something I did during exercise in general: I didn't breathe well. I'd get through a part of a workout or something, and then notice that I was really short of breath. It's something I still need to pay attention to because otherwise, I won't get anywhere near enough air. When I focus on my breathing while running, I definitely have a sense of control and ability. It helps me feel everything that's happening, really: I then naturally notice my legs ("Hey! I'm running!"), my feet ("Good pick on the shoes!") and my chest ("YOU REALLY NEED TO BREATHE!")
Relaxing one's breathing while running is really hard to do.
Of course, during yoga this is essential, as it is a form of meditation. Breathing is coordinated with movement, and doing one without the other leads to imbalance. It's fascinating to me to see the two working together in myself.
One last thing about breathing.
Early into my exploration of meditation I read Zen Mind, Beginner's Mind (absolutely phenomenal) and Suzuki talks of the importance of breathing.
Why is breathing so important? Concentrating on our breathing reminds us that we are totally dependent on the world around us, on the very air we breathe. It also reminds us that if we are breathing, then we are alive and therefore independent. If you realize the fact of this dependence/independence, it can free you. We are not talking of some intellectual idea, but a very real, physical thing.
The most powerful thing I remember about breathing is that it is like a swinging door, and inhalation brings the entire world in just as exhalation brings the entire world out. (Although "in" and "out" are arbitrary concepts... I trust you get the idea). I focus on everything "outside" of me when I breathe, and how when I'm doing it, I'm taking everything in. All of it. The entire universe. And when I breathe out, I'm letting everything out. All of it. The entire "me".
Breathing ultimately starts being about me and what I'm doing, but soon becomes not about me at all. It just is.
And yeah, driving feels pretty inconsequential as a result of this.