About breathing during zazen, Dogen Zenji said in The collection of Dogen Zenji's formal speeches and poems (Eihei-koroku), vol. 5: ...
In the Small Vehicles [of which Dogen disapproved}, there are two elementary ways (of beginner's practice): one is to count the breaths, and the other is to contemplate the impurity (of the body). In other words, a practitioner of Small Vehicles regulates his breathing by counting the breaths. The practice of the Buddha-ancestors, however, is completely different from the way of Small Vehicles. An ancestral teacher has said, “It is better to have the mind of a wily fox than to follow the way of Small Vehicle self-control.” Two of the Small Vehicle schools (studied) in Japan today are the precept school (Shibunritsu) and the school based on Abhidharma-kosa (Kusha).
There is also the Mahayana way of regulating breathing. That is, knowing that a long breath is long and that a short one is short. The breath reaches the tanden and leaves from there. Although the exhalation and inhalation are different, they both pass through the tanden. When you breathe abdominally, it is easy to become aware of the transiency (of life), and to harmonize the mind.
My late teacher Tendo said, “The inhaled breath reaches the tanden; however, it is not that this breath comes from somewhere. For that reason, it is neither short nor long. The exhaled breath leaves from the tanden; however, it is not possible to say where this breath goes. For that reason, it is neither long nor short”. My teacher explained it in that way, and if someone were to ask me how to harmonize one's breathing, I would reply in this way: although it is not Mahayana, it is different from the Small Vehicle; though it is not of the Small Vehicles it is different from Mahayana. And if questioned further regarding what it is ultimately, I would respond that inhaling or exhaling are neither long nor short.
http://global.sotozen-net.or.jp/eng/how ... zazen.html