The short version
Lips closed, jaw loose, an easy sustained mmm on a comfortable pitch, run to the natural end of one exhale. 3 to 5 of them, no forcing the volume. A clamped throat can’t hum comfortably, which is most of the point. For re-anchoring after something has gone wrong.
What the research says
Humming forces a long exhale, and the long exhale is the lever. When researchers had people hum with their ears blocked so they couldn’t hear the sound, nothing changed; the effect tracked the breathing rate, not the vibration. The famous nitric-oxide spike is real and was proposed as a diagnostic test, not a therapy. Cited under Chapter 6.
Rooms where it does the work
Two rules, no exceptions
Never practice any technique on this site while you are driving, operating machinery, or doing anything that needs your full attention. The slow ones can make you drowsy, faster than you’d expect. And never practice a breath-hold, bellows breath, or any vigorous breathing in or near water. Read Before you begin for the doctor’s list and the rest.
The short version is a reminder, not a lesson. The chapter has the story, the steps, the mistakes, and the limits.