Yoga instruction ties movement to breath more consistently than almost any other physical practice. That link is doing specific work, both mechanical and neurological.
Breath sets the tempo
When a movement is assigned to an inhalation or an exhalation, the duration of the breath determines how long the movement takes.
Slowing the breath slows the sequence, increasing the time spent under load and changing a flowing class into a considerably harder one without altering the postures.
This gives an instructor a single control that adjusts intensity for a whole room, and gives a practitioner a way to self-regulate within a shared sequence.
The nervous system responds to the exhale
Heart rate rises slightly during inhalation and falls during exhalation, a pattern driven by vagus nerve activity that varies across the breathing cycle.
Extending the exhalation lengthens the period during which that slowing influence dominates, which is measurable as a change in heart rate variability.
Practices that emphasise long exhalations are working through this route, which is why they produce a settling effect that faster breathing does not.
Mechanics constrain which postures suit which phase
Inhalation expands the ribcage and lengthens the spine, which fits movements that open the front of the body.
Exhalation draws the ribs down and allows the abdomen to draw in, which suits forward folds and twists where compression is required.
The conventional pairing of breath to movement follows from this rather than from convention alone, and reversing it makes many postures physically awkward.
Breath acts as a limit indicator
Losing the ability to maintain the assigned rhythm signals that the effort has exceeded what can be sustained.
Instructors use this as a practical guide, since it responds to the individual rather than to an external standard of how a posture should look.
Holding the breath during a difficult position raises pressure inside the chest and abdomen and briefly raises blood pressure, which is why it is discouraged.
Formal breathing practices are separate
Techniques involving forceful exhalation, alternate nostril breathing or extended retention are distinct disciplines with their own effects, usually taught apart from the postures.
Some produce substantial swings in blood chemistry and are traditionally taught with progression and supervision rather than attempted from a description.
Anyone with high blood pressure, a respiratory condition, epilepsy or a pregnancy should raise intensive breathing practices with a clinician, since the physiological effects are real enough to matter.