The short version
Let the belly expand on the inhale instead of lifting the chest and shoulders. The default posture underneath almost every other technique in this book.
What the research says
Standard respiratory physiology; nothing to overclaim. Nasal breathing’s warming, filtering, and nitric-oxide effects are real and small, with the two foundational studies (six people each) cited under Chapter 14.
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.