Breathing is the only vital system you can voluntarily control. When you inhale slowly through the nose, inflating the belly, the diaphragm descends and stimulates the vagus nerve, the direct line between brain and relaxation system. Five breaths (inhale 4 seconds, exhale 6) and the shift occurs in under two minutes.
Cardiac coherence, breathing at six cycles per minute for five minutes, is the most studied application. Results are consistent: reduced cortisol, lower heart rate, decreased muscle tension. Not mystical. Nervous system mechanics.
For chronic pain sufferers, breathing is particularly valuable. Pain activates the alert system, which contracts muscles, which amplifies pain. Controlled breathing interrupts this cycle.