In order to breathe out into the shakuhachi,you first have to take a breath in. How you breathe in affects how you breathe out.
We often hear that abdominal breathing is good, but it is more complicated than that.
[Translator's note: Abdominal breathing is when you breathe such that your abdomen/stomach area expands on the in-breath,whereas chest breathing is where your lungs/chest expand on the in-breath. Try placing a hand on your tummy to see which you are doing. If your hand bulges outward when you breathe in, you are breathing abdominally; if it draws in closer to your body on the in-breath, you are chest breathing. Abdominal breathing elicits a relaxation response too, by the way.]
The difference between abdominal or chest breathing is in how your lungs expand and contract. In abdominal breathing,your lungs expand vertically as your diaphragm draws them downward. In chest breathing, your lungs expand in/out - they open up like French doors when you breathe in.
The crucial difference here is how much control you have during out-breaths. The fact is that you have much more control over out-breaths in abdominal breathing because you have more muscles in that area to control the action. This means that abdominal breathing gives more control for long, soft notes, or sudden loud ones.
So, the question becomes: How do you get better at abdominal breathing? I mentioned above that more muscles are involved in abdominal breathing. That means that making it slightly more difficult to take an in-breath encourages abdominal breathing.
For example, try breathing in with your mouth wide open, then through your nose. You are likely to find that you naturally do chest breathing in the first instance and abdominal breathing in the second.
There are many reasons why you should not breathe in through your mouth playing shakuhachi: losing your optimal angle or drying out your mouth are just two of these. Now we can add making it more likely to chest breathe to the list. This is why you should breathe through your nose.
However, sometimes breathing in through your nose can take too much time. So, one technique I use is to begin breathing in through my nose (abdominally), then continue and finish the breath through my mouth. It is an interesting phenomenon that even if you switch mid-breath from your nose to your mouth, it will stay an abdominal breath.
Try it yourself, such as in your daily Ro practice. It won't come immediately, but eventually you will find your selfbreathing both abdominally and quickly if you start the in-breathe from your nose and then continue it through your mouth.