An illustrated menu of breathing options from the NYT:
Consciously changing the way you breathe appears to send a signal to the brain to adjust the parasympathetic branch of the nervous system, which can slow heart rate and digestion and promote feelings of calm as well as the sympathetic system, which controls the release of stress hormones like cortisol.
Many maladies, such as anxiety and depression, are aggravated or triggered by stress. “I have seen patients transformed by adopting regular breathing practices.”