Here’s a link to a short article on how breathing facilitates stress release.

This article says that rapid shallow breathing, as in fire breath, stimulates the energizing sympathetic nervous system, while slow deep breathing stimulates the calming parasympathetic nervous system.

I’d heard previously that emphasizing inhalations stimulates the SNS, while emphasizing exhalations stimulates the PNS.

I do know that long, slow exhalations are very calming.

Just Breathe: Body Has A Built-In Stress Reliever : NPR.