FREEDIVING EQUALISATION MASK
When freediving, the mask gets pressed against the face. pressure is applied on the soft skirt of the mask and reduces its internal volume.
This phenomenon can be avoided by equalising the internal pressure of the mask and the external pressure exerted by the deep water.
Simply breathe out gently through the nose once after the first few metres.
This equalisation action is performed sparingly during the descent to save air in the lungs.