Your mask creates an air space in front of your face that can squeeze when pressure increases. You must prevent a squeeze by equalizing the pressure inside the mask as you descend.

Equalizing your mask is accomplished by exhaling a small amount of air through your nose, and this is why a nose pocket is required. Equalization usually happens automatically as you breathe, but if you feel pressure during descent you may need to consciously exhale through your nose to equalize the pressure.

Failure to equalize your mask can cause your eyes to become bloodshot. If this occurs, you should have your eyes examined by a doctor, and avoid diving until your eyes have healed.