Getting a good shakuhachi tone often requires a lot of air pressure. How do you get this pressure without expending so much air that your breath is very short?
The trick is to hold back the air in two places: the embouchure (lips) and the abdomen.
Embouchure: A good embouchure is one in which the opening between the lips is small enough that not much air is wasted and a pressurizing "jet" is created. If you shape your embouchure effectively, you will get a sense of the breath being held back (conserved) even when you blow very hard. Experiment with your embouchure, trying to find not only the "sweet spot" where you get a nice pure tone but also the "sweet pressure" where you can feel a significant pushing back against your breath. Try a more horizontal embouchure, one that lines up with the utaguchi. As you experiment with your embouchure, make sure that your cheeks and face stay relaxed. The way to test this is to make sure that you can let your cheeks puff out Dizzy Gillespie-style even when your embouchure is very refined.
Abdomen: As you breathe out fully and forcefully, try to hold back the air with your abdominal muscles. Try and feel a sensation of inhaling with those muscles even as you exhale. Create a dynamic in your abdomen between breathing out and holding back. This may be difficult to feel at first, but keep working with it. Try to extend the length of your breath, starting with thirty seconds and gradually working your way up to a full minute or more -- all by holding back with your abdomen.
These two techniques will enable you to use a strong, full breath rather than conserving your air simply by blowing more gently. A strong full breath is the basis of a good shakuhachi tone.