Hi m m a -
I've been moving this book around the books' shelves since last reading from it in Summer 2005, even returning it to the used book store for a year at one point and retrieving it when no one bought it. So, since i'm in a bit of a funny place dealing with persistent and perceived thoughts I spent a few hours practicing it this afternoon. I sat zazen a long time ago and was not taught this technique, however this may be because our group lacked a monastic teacher for many years and thus no one to teach something like this.
If this phrase, "Bamboo breathing" is foreign, I could type up the passage from the book.
What passage is giving you issue? You can state the page if you like.
I was wondering if anyone else could provide commentary or an alternate description.
Essentially, the diaphragm is
the means of lung-filling.
Lung-filling happens at a rate determined by one's personal urge to re-establish a reserve lung volume. When our reserve lung volume is pretty high we do not gasp for air. When that reserve is very low (residual volume), we gasp for air whether we want to or not.
On a normal inhale, one is primarily
i) breathing from the upper thoracic cavity (as if air is being pulled up from the ribs through the clavicles) and the clavicles are felt to rise, and muscles between the ribs press outwards, (the diaphragm drops a bit) or
ii) breathing is happening from first the abdomen, then up through the thoracic cavity.
This ii) style of normal breathing is something like if two hands are placed just below the xiphoid process (pointy bone at the center of the front rib cage where ribs come together) and gently spreading apart the torso like spreading bread dough in opposite directions on a board. The abdomen drops naturally, then chest rises naturally - lungs are filled. The exhale is more symmetrical like stove bellows - the two parts are coming together more evenly: chest muscles squeeze a little (rib cage narrows) and belly button may retract.
*
So the abdominal breathing Sekida suggests involves preventing the upper chest from causing the lungs to fill. The clavicles and neck area do not rise much, if at all, and the muscles between each rib are expand
ed as a result of the lungs' expansion - they are not causing the lungs to expand (remember: in abdominal breathing, the contracted (dropped and down-pressed diaphragm) is pulling the lungs down and causing them to fill with air). So the rib area is fairly still.
In Part 1 of the bamboo inhalation, the abdomen naturally drops and extends a little, however for Part 2 of inhalation, just as the thoracic cavity seeks to lift up and to pull in air, the diaphragm is specifically contracted (its contraction is a downward movement pulling to the sides) and this prevents thoracic-controlled inhalation. This Part 2 diaphragmatic inhalation pulls the lungs down to fill them. There is tension in the abdomen on the second half, because the diaphragm is contracting and crowding the viscera.
In Part 1 of the bamboo exhalation, the abdomen naturally releases some air, but again, in Part 2, just before the thoracic cavity seeks to squeeze its own muscles and press air out of the lungs, the diaphragm is contracted again and the abdomen and diaphragm press into each other (a little like a well-matched air-wrestle) allowing a slow, thin exhale.
Sekida mentions again and again the tension in the abdomen. He cautions about tension in the chest and concavity of the stomach. I experienced both of these faulty conditions in the first efforts.
Over a few sits my abdomen seemed to be making small contractions like a jellyfish undulating in place and pointing down towards the pelvic cradle. The abdomen looked and felt taut (this is his "keeping tension in the danten").
I can see where the long exhales can lead to what he calls "off-sensation" and further enforce concentration, though I will commonly enter this "off sensation" though with normal, shallow breathing and at any point in the day when there is already immersive concentration. In practicing this breathing again and having experienced some of the conditions of pure consciousness without it, it is unlikely that I would keep it up, but best wishes in your use of it.