“Briefly, the basic principle is:
Holding your breath, shortness of breath, constrained and tense breath, rapid breath = survival, fight/flight/freeze, activation of the nervous system, stress.
Natural, full, diaphragmatic breaths = opening up to life beyond survival, opening up to calmness, regulation of the nervous system, bringing about physical relaxation, improving health, being able to think.
For PTSD specifically, conscious breathing seems to act as a nervous system interrupter. The survival brain gets stuck in this pattern: stimulus in environment (trigger) leads to automatic response of autonomic nervous system (shallow breathing). Taking control of the breathing interrupts this circuitry and breaks the cycle.
Here’s a quote for ya: “Of all the various functions of our autonomic nervous systems, from heart beat, perspiration, hormonal release, gastrointestinal operation, neurotransmitter secretion, etc., the breath stands alone as the only subsystem the conscious mind can put into ‘manual override’ and so it is through manipulation of the breath that we can recalibrate the entire system.” (source)”