Thinking about your question a bit more, you're really digging into anapanasati (mindfulness of breathing) here, which is the main meditation method the Buddha actually taught. The first 4 steps of the
Anapanasati Sutta (MN 118) go like this:
"Now how is mindfulness of in-&-out breathing developed & pursued so as to be of great fruit, of great benefit?
"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.
"[1] Breathing in long, he discerns, 'I am breathing in long'; or breathing out long, he discerns, 'I am breathing out long.' [2] Or breathing in short, he discerns, 'I am breathing in short'; or breathing out short, he discerns, 'I am breathing out short.' [3] He trains himself, 'I will breathe in sensitive to the entire body.' He trains himself, 'I will breathe out sensitive to the entire body.' [4] He trains himself, 'I will breathe in calming bodily fabrication.' He trains himself, 'I will breathe out calming bodily fabrication.'
So that's what you do - you watch the breath and observe the difference between long breaths and short breaths, seeing how they condition the body and learning to calm the body. A good manual for this stuff is Bhikkhu Buddhadasa's
Mindfulness With Breathing:
.
The breath-body is very important because it sustains life in the rest of the body. This relationship is crucial to our study. Although we lack the ability to control the general body, or flesh-body, directly, we can master it indirectly by using the breath. If we act in a certain way toward the breath-body, there will also be a specific effect upon the flesh-body. This is why we take the breath as the object of our training. Supervising the breath, to whatever degree, is equivalent to regulating the flesh-body to that same degree. This point will be more clearly understood when we have trained up to that particular stage of anapanasati.
Beginning our practice with the kaya (body), we study the breath in a special way. Every kind of breath is noted and analyzed. Long breaths, short breaths, calm breaths, violent breaths, fast breaths, slow breaths-we learn to know them all. We examine the nature, characteristics, and functions of each kind of breath that arises.
We should observe the influence of the different breaths upon the flesh-body. We need to see clearly the great effect that the breath has on the physical body. We observe both sides of this relationship until it becomes clear that the two, the flesh-body and the breath-body, are interconnected and inseparable. See that the breath-body conditions and affects the flesh-body. This is the first step. We make a special study of the breath and come to know the characteristics of all its different forms. In this way, we gain insight into the conditioning effect it has on the flesh-body. This, in turn, will allow us to master the flesh-body by means of regulating the breath.
The purpose of these beginning steps is to know the secrets of the kaya, the body. We know that the breath-body, our breathing, conditions the flesh-body. This important secret helps us to use the breath to gain mastery over the body. We discover that by making the breath calm we can relax the flesh-body. If our breathing is calm, the flesh-body will be likewise. Thus, we can regulate our body indirectly through our breathing. Furthermore, we learn that through calming the breath-body and the flesh-body we can experience happiness, joy, and other benefits.
