The double inhale then long exhale which is the Physiological Sigh takes advantage of the second inhale, even if it’s just a tiny bit more air. You got millions of little sacks in your lunges (that makes the area of your lunges as big as a tennis court) and carbon dioxide builds up in those little chambers when we are stressed. This makes us agitated.

The double inhale will reinflate those little sacks and the long exhale is now much more effective at getting rid of the carbon dioxide in your blood. This will calm you down very quickly.

You can do it 3-4 times for it to be efffective

Master Stress, Huberman Lab, 31mins.