I don't see any guide or any real description of the benefit of training your diaphragm (other than taking deep breaths by pushing your stomach out. But how much? How often?)
I train my diaphragm for singing, not really for any other benefits. But I think taking long, deep breaths is great for calming down and finding focus if you're stressed out.
Don't push your stomach out. Let the stomach be pushed out when you breathe in. There's a difference.
The simplest way (I think) to find a 'deep'/connected breath, is to breathe slowly all the way out, making a 's' sound, until there is no more air left. Then, hold for about 4 seconds. Now just relax and let the body breathe in on its own.
For some reason I find that my body doesn't seem to want to breathe in on its own. Like, I can't determine a comfortable point (and I've paid a lot of attention to this over my years in meditation) where my body seems to "take over" from my conscious control and decide to breathe in comfortably on its own. I've found that I hold my lungs empty for longer than is comfortable, to the point where breathing in causes a noticeable rise in heart rate. I'm mildly asthmatic, I'm sure that has something to do with it.
The only way to hand off control of your breathing to your subconscious is to do something that distracts your conscious mind. You can't focus on breathing and then say, "here you go, brain, handle it."
I don't think that's true. When I started breathing meditation, it felt like that for me: whenever I was aware of my breathing, I automatically switched to directly controlling the breath. As a result, my rhythm was off and I often became lightheaded and stopped.
At some point you learn to separate action and observation: you can focus your attention on the breath, meaning the sensations associated with breathing (air flowing through your nostrils and over your upper lip, belly slowly moving), and still let your body control the breathing action automatically.
In The Mind Illuminated, there is a comparison of this to catching a ball: instead of seeing the ball and trying to directly control your arm to catch it, you focus on your intention ("Catch the ball") and let your body simply act on (implement) this intention.
The great thing is that while you practice this separation of action and observation using the breath, it will apply generally to your life. Very powerful for managing your emotions: instead of being your emotion when you become aware of it, and identifying with the urge to act on it, you learn to observe the emotion without that observation having direct effects on your actions. You can choose to act on it, or choose not to.
I did some quick googling and it seems like there's some preliminary research that indicates it might help you reduce stress[0]. But I wonder how this stacks up against going for a jog or riding a bike, which have a well established record of reducing stress, and have other benefits.
It is like breathing more in sync with your center of gravity, it stabilizes you, which makes you calmer, may stimulate your digestive functions as well.
Big belly breaths (when you fill up with air, then top it off, then top it off again) stimulate the vagus nerve, which for me is crucial in combatting my anxiety.
