Usually when we breathe, we breathe in and, as soon as we have finished breathing in, we immediately start breathing out. And as soon as we have finished breathing out, we start breathing in again. There is never any space or gap in between the in-breath and the out-breath. Now, many different ways of focusing the mind on the breathing have been taught.... There are basically six methods taught in the abhidharma. But here we have something different from any of those. This is called gentle threefold breathing. It is called gentle because there is no particular attempt to manipulate the breathing, except that instead of breathing in and then immediately breathing out, after breathing in, you wait before you breathe out...here the duration of the inhalation, of the retention, and of the exhalation should all be equal, three equal periods within each complete breath.
In doing this, some people combine the phases of the breath with the mental repetition of the three mantra syllables: OM AH HUM (HUNG)--OM coordinated with the in-breath, AH with the retention of the breath, and HUM (HUNG) with the out-breath. But what is most important here is simply to recollect, as they occur, the inhalation, retention, and exhalation, so that, while you are inhaling, you are aware that you are doing so; while you are retaining the breath, you are aware that you are doing so; and while you are exhaling, you are aware that you are doing so. In the beginning, it is recommended that beginners start with doing, for example, twenty-one of these breaths as a series, and it is important to practice with enough mindfulness so that, while you breathe in, and so forth, you maintain an awareness of what part of the breathing process you are in.
--from The Ninth Karmapa's Ocean of Definitive Meaning, by Khenchen Thrangu Rinpoche, edited, introduced and annotated by Lama Tashi Namgyal, published by Snow Lion Publications