Yoga instruction returns to the breath more often than to any other single element, which can seem excessive to newcomers focused on the shapes. The breath is doing several structural jobs that the postures alone cannot.
It sets the pace of the practice
Linking movement to inhalation and exhalation ties the speed of the sequence to something with a natural rhythm rather than to intention or ambition.
This prevents the practice from accelerating as attention drifts, which is the most common way a physical routine loses its precision.
It also synchronises a group without anyone counting, which is why led classes remain coherent without constant timing instruction.
It reports on effort more honestly than sensation does
Breathing becomes strained before most people notice that they are working too hard, particularly in a posture where determination overrides discomfort.
The instruction to work only to the point where breathing remains steady converts an internal judgement about intensity into an observable signal.
This is why teachers ask about the breath when a student appears to be straining, rather than asking how the posture feels.
Breathing is both automatic and voluntary
Respiration is one of the few processes that runs without attention and can also be taken over deliberately, which gives it an unusual position.
Voluntary changes to breathing pattern influence physiological variables that are not otherwise under direct control, including heart rate variation.
A slow exhale in particular is associated with a shift towards the branch of the nervous system that dominates during rest.
Posture and breathing constrain each other
The mechanics of breathing depend on the ribcage and diaphragm having room to move, so a collapsed or over-compressed position restricts it directly.
Using breath quality as a check therefore provides information about alignment, since a position that shuts down the breath is usually badly organised.
This is the practical basis for adjusting a posture until the breath is available again rather than until the shape looks correct.
Formal breathing practices are a separate matter
The techniques classified as breath regulation in the tradition include forceful and retentive practices that are quite unlike the steady breathing used during movement.
These have specific effects and traditional cautions attached to them, and are conventionally taught individually rather than picked up from general instruction.
Anyone with respiratory or cardiovascular conditions, high blood pressure, or who is pregnant should establish suitability with a clinician and an experienced teacher before attempting them.