Breath work is using the breath on purpose to change state. Slow nasal breathing to downshift. A physiological sigh when you are spun. Hof-style big ventilation plus a hold for a jolt, on a sofa, never in water. These are different drills. Mixing them in a pool is how people black out.
Nestor is the why (nose). McKeown is the daily nasal work and BOLT score. Hof is the swinging protocol. Device work (The Breather, Powerbreathe) lives on respiratory muscle training. Parent page: breathing and cardio.
Pick one drill
Downshift: in through the nose, longer exhale, two minutes. Alert: cold shower, or a short Hof round sitting down, then normal breathing before you stand. Sleep: nose at night if the airway allows it; tape only if you can still breathe. If the nose is blocked, fix the nose first.
FAQ
Is this cardio?
No. It changes autonomic state. Zone 2 still has a job.
Can I do Hof in the bath?
No. Hyperventilation plus water is a drowning protocol.
How often?
The downshift can be daily. The big swinging round a few times a week is plenty.
Not a treatment for panic disorder or asthma without a clinician. Chest pain or fainting: stop.