Internal volume
The volume of air trapped in the mask, between the face and the glass, represents the internal volume. The smaller the internal volume, the easier it is to balance the mask during apnea descents.
Compensation
Mask plating occurs on descent. As soon as the mask is immersed, the pressure presses on the mask's flexible skirt, reducing its internal volume and pressing the mask against the face. To avoid this phenomenon, which is painful for the eyes, it is important to compensate for the internal pressure of the mask and the external pressure exerted by the water. To do this, simply blow slightly through your nose. Air is so precious in these sports that the compensating action should be minimal.