What problem mouthfill solves
As depth increases, lung volume decreases. A mouthfill moves a reserve of air into the mouth and upper airway before access to lung air becomes difficult, then that reserve is managed for the remaining equalizations.
Frenzel comes first
If Frenzel is inconsistent, mouthfill usually adds another layer of complexity rather than solving the real problem. Stable tongue, glottis and soft-palate control should already be familiar.
The charge
The diver brings air from the lungs into the mouth at a chosen depth while there is still enough accessible volume to do so comfortably. The exact timing is individual and depends on lung volume, technique and target depth.
After the charge
The glottis stays closed so the stored mouth air is preserved. The diver then uses small, controlled tongue movements to deliver only the pressure needed for each equalization.
Why mouthfill collapses
Common causes include charging too late, leaking air back toward the lungs, losing soft-palate position, wasting too much air per equalization, tension in the jaw and cheeks, or simply descending faster than control allows.
Depth is not the learning metric
A better measure is whether the diver can charge predictably, keep the mouthfill stable, equalize without strain and abort early when coordination is lost.
Learn with an instructor
Educational articles can help you understand the concepts, but equalization and safety skills should be developed progressively with direct feedback in an appropriate training environment.
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