I am pretty sure it has a direct mechanical nervous system stimulus. https://www.google.com/amp/s/www.psychologytoday.com/us/blog/the-athletes-way/201705/diaph

I am pretty sure it has a direct mechanical nervous system stimulus. https://www.google.com/amp/s/www.psychologytoday.com/us/blog/the-athletes-way/201705/diaphragmatic-breathing-exercises-and-your-vagus-nerve%3famp https://www.google.com/amp/s/www.psychologytoday.com/us/blog... It also hits your HRV. My Garmin watch (Fenix 5X) has a deep breathing timer (relax timer). It will definitely increase my HRV when I do it (lowering the watches real time stress score, which is measuring HRV). So it is physiological and psychological.