The short version
In for a count of 4, hold for 4, out for 4, hold for 4, through the nose. Repeat 4–6 times. If holds make you air-hungry or panicky, drop them and use 4 in, 6 out instead. Invisible under a table. Good for sustained pressure you have to sit through.
What the research says
Widely taught in military and emergency-services training; it performed comparably to cyclic sighing in the Stanford trial but has been studied far less as a named protocol. If holds make you air-hungry or panicky, drop them and use four in, six out.
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.