Breathing in and out.
When you want.
As fast or slow as you want.
As hard or soft as you want.
For as long as you want.
Whenever you want.
But what does it even mean? Well, basically, we humans can breathe through our nose and/or mouth by using our lungs and/or our diaphragm to pump air in and out. Our lungs are up in our chests, and our diaphragm is underneath, about the level of our stomachs. Learn to isolate and separate the feelings of breathing from your lungs, both your lungs and diaphragm, and just your diaphragm.
The common advice on how to go about isolating your diaphragm breathing is to lie on the floor (hard surface helps) on your back and relax. Put a book on your stomach and watch it move up and down. Feel the weight of the book against your stomach and feel where your stomach pushes back against it. Down there. That's your diaphragm. Breathe from there. Breathe all the way out and cough. Feel the pounding of your diaphragm?
Get your lungs out of it. If you feel your chest expanding you're breathing from your lungs. Chest equals lungs. Stomach equals diaphragm. Some guys say "play from your toes." From deep. As deep as you can get. If your shoulders are going up and down you're breathing from your lungs. Lungs are higher up. Feelings caused by breathing that are high up in your body--your chest and your shoulders--are caused by breathing from your lungs. Relax those muscles. Stop expanding your chest and raising your shoulders.
Why? You don't like the answer that experience shows that breathing from the diaphragm works and sounds best? You're not happy with the thought that all the great players say the same things, and that the same wisdom is known by singers? Okay, here's another justification for you.
It turns out that our mouths are not quite big enough to provide an optimally sized resonant echo chamber for the sounds we produce either playing or singing, especially on lower notes. The mouth is very important in manipulating the resonant chamber size (and shape) so we can tune it to the notes we are playing, reshaping the sound, but we need a little bigger chamber, something with more volume. We add our hands, and this helps both in creating a larger chamber and providing a way to manipulate the chamber size to maximize the impact of each individual note. But hands and mouth together are still not big enough. The control comes from there, but the capacity--the size and volume of the chamber in which the sounds echo and resonate, benefits from use of the entire vocal tract, everything below the throat and back down into the lungs. The lungs themselves are full of tissues and fluids, along with a myriad air sacks, so I don't know how much the lungs themselves add to the resonance chamber--but, the airways that lead to the lungs are very important. We need to relax the chest muscles that operate the lungs so we can control and expand the vocal tract resonance chamber. Then we can still use the diaphragm to control the air flowing into and out of our lungs while maintaining control over the vocal tract that feeds the lungs. We need to make ourselves big on the inside to take advantage of the resonant capabilities of our bodies.
What do you do if you run out of air, either breathing in or breathing out? Well, pretty much you have to breathe out or in to compensate, if only a little. If your mouth is on the harp and you change your breath direction, you change the note you're playing as well, and if you don't want to do that you have two choices: breathe through your nose or remove your mouth from playing position.
You can quickly take a bite of air, or let some go, by momentarily taking your lip off the cover plate and breathing through your mouth but not through the harp. This feels much like a biting motion, where you're biting at the harp. This lip biting technique can be used to articulate notes, and the articulation can be combined with breath equalization.
More often, excess air is expelled through the nose while playing notes while breathing out. Some players talk about almost always letting a little air out when they play blow notes because they know they'll need the lung capacity later, since they play mostly draw notes. Running out of air is seldom the problem. Having too much air is often the problem, and this letting out of air during blow notes helps keep their lungs in equilibrium.
On the other hand, air leaking in through the nose during draw notes is usually not a good idea. Control over draw bends and vibrato is greatly reduced or lost if air is allowed to come in the nose. And besides, as I said above, too much air is usually the problem, and excess air coming in is not what you need. If you have trouble holding notes for a long time, see if you are letting air enter or escape through your nose. If you are, you need to learn to control that. Don't get discouraged if you can't control it right away. Like so many things, it's something you have to work at and learn. But once you get it you probably won't even think about it again, unless you read something that reminds you...
Anything that helps you gain relaxation control over your breathing should be beneficial to your harp playing.
Meditation in its various forms and guises tends to emphasize controlled, often slow and deep breathing, sometimes even with chanting. Breathing exercises associated with meditation practice can be applied to breathing for the harmonica.
Running, swimming, and other aerobic exercise can help strengthen your lungs and possibly improve your lung capacity, or at least allow you to more effectively utilize the lung capacity you have.
Most people breathe from only the top third or so of their lungs. Most of the capacity lower in the lungs is not used, not well developed, and not contributing much to the person's breathing. Playing diatonic harp in cross harp position (2nd position) involves mostly breathing in. When you "run out of air" it's usually because you are getting too much air into your lungs. We need to develop the lower parts of the lungs, breathing fully and deeply.
Breathe out slowly as far as you can. Completely empty your lungs. Squeeze out the last bits of air. Got it all out? Try coughing--see how much more there was? Cough some more. Get all that old deep stale air out of there.
Then slowly and willfully allow your lungs to begin filling. This is a process of gradually relaxing the muscles that have been called into play to force the air out, up to a point. After a while your lungs will be comfortably full of air, but keep slowly and gently filling them deeply. Concentrate on using your diaphragm to take your slow deep breath. It should feel like you are pushing out with your stomach. Make sure your chest is not expanding and your shoulders aren't rising. Take several such long slow breaths, completely emptying your lungs first, then relaxing with control to slowly fill them back to neutral, then drawing with your diaphragm to fill them completely full. Think you've got them all the way full? Try a few quick inward pants just when you think you're holding as much air as you can. Picture the videos you've seen of pot smokers trying to inhale just a little bit more.
Pant like a dog. Go on. Feel the exercise your diaphragm is getting? Open yourself up and go to it. Let out all your air and pant. Fill your lungs full and pant. Keep panting while slowly breathing all the way in and all the way out. That's not as easy as it sounds, is it.. Keep at it, you'll be gaining strength and control.
Practice rhythmic breathing.
Practice train songs.
Practice holding notes for a long time, both breathing in and breathing out.
While you're holding notes, slowly vary the volume from soft to loud to soft. Gain control of how much pressure you are using to generate the different dynamics.
Practice with the harp in your mouth, and play big full round smooth even chords.
Practice without the harp in your mouth when you can think about breathing, but can't play your harp.
Playing the harp
Begins and ends
With breathing.
Breathing in and out.
When you want.
As fast or slow as you want.
As hard or soft as you want.
For as long as you want.
Whenever you want.
Work on all those things.
