The trapped air inside the mouth and nasal cavities is compressed by the movement of the tongue or larynx while doing Frenzel maneuver. The air is forced into the nasal cavity by the pressure and tries to exit by the nose, but the nostrils are squeezed shut. Because the glottis is closed, air cannot return to the lungs. The tongue creates an airtight seal against the upper teeth or in the rear of the mouth, preventing air from escaping. Because there is nowhere else for the air to go, it enters the eustachian tubes and the middle ear, equalizing the pressure.[4]
The Frenzel Maneuver is performed as follows:[2][3]
the way out of the nasal cavity is blocked, typically by pinching the nostrils or by a nose-clip;
the way out of the mouth cavity is blocked by the tongue, either performing an alveolar closure (as when the sound [t] or [d] is produced; this is the standard version) or a velar closure (using the back of the tongue; this is the advanced version, leaving very little possibility for the piston movement by the tongue, mentioned below);
the larynx is closed, sealing off the lungs;
the body of the tongue (or only the back of the tongue, if velar closure has been made) is gently moved upwards and backwards, in order to compress the air;
further compression of the air follows from moving the closed glottis upwards (this is easily noticed in the elevation of the "Adam's Apple").
By performing this technique, the compressed air is forced into Eustachian tubes, and thus into the middle ear. In situations where the ambient pressure rises (typical causes are decreasing altitude in the case of an airplane or increasing depth in the case of a diver submerging), the maneuver results in the equalization of the pressure on both sides of the eardrum.[2][3]
^Molvaer, Otto I (2003). Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. pp. 232–3. ISBN0-7020-2571-2.