In this section, the Buddha outlines sixteen ways of approaching the Mindfulness of Breathing meditation practice.
Now how is mindfulness of in-and-out breathing developed and pursued so as to bring the four foundation of mindfulness to their culmination?
There is the case where a monk, having gone to the wilderness, to the shade of a tree, or to an empty building, sits down folding his legs crosswise, holding his body erect, and setting mindfulness to the fore. Always mindful, he breathes in; mindful he breathes out.
The first tetrad (group of four practices) relates to mindfulness of the body — first just being aware of the breathing (which is a physical sensation) and noticing whether the breaths are short or long. Then the breathing is noticed in relation to the entire body, so that the practice is no longer narrowly focused on the sensation of the breathing, but instead the breathing is used as an anchor for a wider sense of awareness. Sometimes we talk about “mindfulness with breathing” instead of “mindfulness of breathing” in order to make this distinction. Lastly, the meditator uses this mindfulness of the entire body to bring about physical calmness and release, or relaxation.
First Tetrad Breathing in long, he discerns that he is breathing in long; or breathing out long, he discerns that he is breathing out long.  Or breathing in short, he discerns that he is breathing in short; or breathing out short, he discerns that he is breathing out short.  He trains himself to breathe in sensitive to the entire body, and to breathe out sensitive to the entire body.  He trains himself to breathe in calming the body, and to breathe out calming the body.
So in this stage (1 and 2 — although we shouldn’t take these 16 approaches to mindfulness of breathing to be stages that we need to work through in order) we’re establishing the first foundation of mindfulness.
We’re noticing first of all simply whether the breaths are short or long. Then (3) we’re using our focus on the breath as a central point of reference to help establish mindful awareness of the whole body. We don’t need to think about being narrowly focused on the breathing in the mindfulness of breathing practice. The breath is just a tool to help us develop mindfulness, and it’s not an end in itself.
Then (4) we’re allowing the body to relax deeply. This isn’t a conscious process of letting go, but the release from within of tensions that have been help unconsciously in the body. From time to time there will be a sense of sudden release, although sometimes there will just be a gradual sense of ease. The body can start to feel more alive and energized, while at the same time remaining calm and still.
This leads into the next tetrad, which is concerned with feelings and emotions. As the body relaxes, energy is released. This energy is known as rapture, and can be experienced in many ways.
Some of the commonest manifestations of rapture, or piti, are rushes of energy, or tingling, or pleasant sensations in various parts of the body. While these sensations arise, the meditator should simply observe them, without becoming elated by them.
One good way to achieve this — and which the sutta recommends — is to be aware of the rapture in relation to the breathing. The breathing is used as an anchor, and this stops the meditator from being blown off course by these often powerful winds of pleasure. As physical energy is released, positive emotions are experienced, and again the breathing is used as an anchor to allow the meditator to absorb these experiences with equanimity. Pleasure and rapture are natural consequences of meditating — they just happen automatically as we start to relax.
The meditator also becomes more aware of how the mind tends to show aversion towards unpleasant sensations and attachment towards pleasant ones. Becoming more aware of these mental processes, we can then let go of craving and aversion and simply experience what we’re experiencing. This leads to the emotional calmness that we call equanimity.
Second tetrad He trains himself to breathe in sensitive to rapture, and to breathe out sensitive to rapture.  He trains himself to breathe in sensitive to pleasure, and to breathe out sensitive to pleasure.  He trains himself to breathe in sensitive to mental processes, and to breathe out sensitive to mental processes.  He trains himself to breathe in calming mental fabrication, and to breathe out calming mental fabrication.
The third tetrad deals with the mind and emotions. As we stop reacting to our experience, we start to become happier. First we notice the mind, and then we notice how the mind becomes more joyful. With the mind having become more joyful, these is less of a tendency for it to wander, and so the mind becomes steadier. Once we have developed concentration in this way, we can then apply our mind to liberating itself, since the mind has become a powerful tool for reflection.
Third Tetrad He trains himself to breathe in sensitive to the mind, and to breathe out sensitive to the mind.  He trains himself to breathe in satisfying the mind, and to breathe out satisfying the mind.  He trains himself to breathe in steadying the mind, and to breathe out steadying the mind.  He trains himself to breathe in releasing the mind, and to breathe out releasing the mind.
The fourth tetrad shows four ways that we can reflect in order to free the mind. In these reflections we contemplate the impermanence of our experience as we notice the arising, enduring, fading, and passing of sensations, feelings, emotions, and thoughts.
This can involve simply noticing sounds arising and falling, or noting how an itch constantly changes, or it can involve watching a complex “story” unfolding in the mind and then dissipating.
Fourth Tetrad He trains himself to breathe in focusing on impermanence, and to breathe out focusing on impermanence.  He trains himself to breathe in focusing on fading and to breathe out focusing on fading.  He trains himself to breathe in focusing on cessation, and to breathe out focusing on cessation.  He trains himself to breathe in focusing on relinquishment, and to breathe out focusing on relinquishment.