Zazen is correctly described as a practice of "body, breath and mind." All come to play in 'Just Sitting.' We have already discussed "dropping bodymind," so now let's talk about "dropping the breath."
Some kinds of meditation practices emphasize doing something unusual with the breath, breathing with special rhythms or with intentional contractions or special depths, in order to bring about unusual mind states. Shikantaza is different, and merely emphasizes breathing during Zazen in a natural, healthful, ordinary way. Oh, we let the diaphragm nicely expand to fill the lungs, but not in some extreme or forced way. Just breathe nice and deep so as to nourish the body ... then forget about the breath. Inhalations reach their natural fullness, perhaps pause for an instant, then exhalation easily and gently begins. Your body will instinctively take care of the doing without your need to do anything.
Just let the breath settle into its own, natural rhythm. Master Dogen said to allow the breath to settle and become ordered. He taught (Eihei-Koroku 5-390):
If following the breath as a point to place attention during Zazen, one may lightly focus on the breath entering and exiting gently through the nose. However, don't particularly think about the breath. We sometimes say that we sit while subtly feeling that our inhalations are reaching all the way down to the "hara" or "tanden," a traditional "energy" center of the body a little below the navel. Imagining so gives a sense of the breath fully flowing through our whole body. But, of course, the breaths are not reaching there directly except in the flowing blood. (If your inhalations are actually somehow escaping the lungs to reach your navel ... SEE A DOCTOR IMMEDIATELY! 
). And as the breath reaches a nice, balanced pace, we forget all about measures of "long or short," as well as "inside and outside." Dogen continued:
Ultimately, the most important aspect of breath is to just forget about it. In doing so, the hard borders of "inside" and "outside" the body begin to soften and drop away. We realize that the breath is the outside flowing in, while exhalations are the inside flowing out, such that inside is just outside in and outside is just inside out. All is thus embodied in the body of Zazen. As inside and outside drop away, the sense of "me" vs. "not me" also begins to soften or fully drop away. Shunryu Suzuki described it so [in ZMBM] ...
So, breathing during Zazen is as simple as that ... as simple as Just Breathing!
And one more thing ... DON'T STOP! Not for very long anyway.
Gassho, J
STLah
Some kinds of meditation practices emphasize doing something unusual with the breath, breathing with special rhythms or with intentional contractions or special depths, in order to bring about unusual mind states. Shikantaza is different, and merely emphasizes breathing during Zazen in a natural, healthful, ordinary way. Oh, we let the diaphragm nicely expand to fill the lungs, but not in some extreme or forced way. Just breathe nice and deep so as to nourish the body ... then forget about the breath. Inhalations reach their natural fullness, perhaps pause for an instant, then exhalation easily and gently begins. Your body will instinctively take care of the doing without your need to do anything.
Just let the breath settle into its own, natural rhythm. Master Dogen said to allow the breath to settle and become ordered. He taught (Eihei-Koroku 5-390):
In the lesser vehicle originally there were two gateways, which were counting breaths and contemplating impurity. In the lesser vehicle, people used counting to regulate their breath. However, the buddha ancestors’ engaging of the way always differed from the lesser vehicle.
). And as the breath reaches a nice, balanced pace, we forget all about measures of "long or short," as well as "inside and outside." Dogen continued:
In the Mahayana there is also a method for regulating breath, which is knowing that one breath is long, another breath is short. The breath reaches the tanden and comes up from the tanden. Although exhale and inhale differ, both of them occur depending on the tanden. Impermanence is easy to clarify, and regulating the mind is easy to accomplish. But my late teacher Tiantong [Master Rujing] said, “Breath enters and reaches the tanden, and yet there is no place from which it comes. Therefore it is neither long nor short. Breath emerges from the tanden, and yet there is nowhere it goes. Therefore it is neither short nor long.”
My late teacher said it like that. Suppose someone were to ask Eihei [Dogen], “Master, how do you regulate your breath?” I would simply say to him: Although it is not the great vehicle, it differs from the lesser vehicle. Although it is not the lesser vehicle, it differs from the great vehicle. Suppose that person inquired again, “Ultimately, what is it?” I would say to him: Exhale and inhale are neither long nor short.
My late teacher said it like that. Suppose someone were to ask Eihei [Dogen], “Master, how do you regulate your breath?” I would simply say to him: Although it is not the great vehicle, it differs from the lesser vehicle. Although it is not the lesser vehicle, it differs from the great vehicle. Suppose that person inquired again, “Ultimately, what is it?” I would say to him: Exhale and inhale are neither long nor short.
If you think, “I breathe,” the “I” is extra. There is no you to say “I.” What we call “I” is just a swinging door which moves when we inhale and when we exhale. It just moves; that is all. When your mind is pure and calm enough to follow this movement, there is nothing: no “I,” no world, no mind nor body: just a swinging door.
And one more thing ... DON'T STOP! Not for very long anyway.
Gassho, J
STLah
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