The short version
Attention resting on the feel of air at the nostrils, cool in and warm out, nothing to count or change. The lightest anchor in the book, for a sit or for any minute you want to stop managing the breath and just keep it company; when the mind leaves, notice and come back.
What the research says
The oldest anchor in meditation and the lightest one in the book. Craft.
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.