In the first minutes after a hard set, sprint, or workout (or after any surge of stress or adrenaline) to bring your heart rate and breathing down and shift out of fight-or-flight faster.
A longer exhale relative to the inhale increases parasympathetic (vagal) activity. During each exhalation the vagus nerve naturally slows the heart (the effect behind respiratory sinus arrhythmia) so emphasising the out-breath nudges heart rate down and moves the body from sympathetic 'go' toward parasympathetic 'recover'. Slow nasal breathing also makes gas exchange more efficient and reduces the feeling of breathlessness. The net effect is a faster, more controlled downshift toward your resting baseline.
Slow, extended-exhale breathing reliably shifts heart-rate-variability and vagal markers in the short term and is well studied as a down-regulation tool. Evidence that it meaningfully speeds athletic recovery (for example between sets) is more limited and mixed, but it is low-risk and the acute calming effect is robust. Use it as a dependable composure tool rather than a proven performance booster.
Deliberate recovery breathing is long-standing practice in athletics, martial arts, and the military as a way to compose the body after exertion, and it overlaps with the long-exhale and pursed-lip techniques used in respiratory clinics. It has no single inventor, it's a convergence of coaching lore and respiratory physiology; the modern 'physiological sigh' popularised through Stanford researchers is a closely related fast-acting variant.
Start by simply making the exhale longer than the inhale after any effort. Build to a steady 4:6, then to 4:8 or 5:8. Fold it into structured rest intervals between sets, and reach for a physiological sigh when you need to settle fast.
Nose or mouth right after a hard effort?
Start with whatever lets you breathe (mouth is fine when you're truly gasping) then move to slow nasal breathing with a long exhale as you regain control.
How is this different from just catching my breath?
Catching your breath happens on its own; deliberately lengthening the exhale actively engages the vagus nerve to bring your heart rate down faster and more calmly.
How long should I do it?
One to three minutes usually gets you close to baseline. Use it for as long as it keeps helping.
Safety
Breathwork is a complement to (not a replacement for) professional medical care. If you have a heart, respiratory, or blood-pressure condition, are pregnant, or feel unwell, check with a clinician before practising breath-hold techniques.