Swimmers and divers can relieve ear pressure through the Valsalva maneuver, the Frenzel maneuver or by swallowing and wiggling the jaw. Air pressure in the ears is regulated by the Eustachian tube which links the middle ear to the throat.
The Valsalva maneuver can be performed by pinching the nostrils shut and exhaling through the blocked nose. The Frenzel maneuver is similar, but while breathing against the pinched nostrils, the swimmer needs to swallow. Finally, a scuba diver can equalize his ears by swallowing or wiggling the jaw while keeping the regulator inside his mouth.
As a swimmer descends, water pressure increases, affecting the outer ear which matches the pressure of the environment. However, the middle ear is sealed, preventing pressure changes. When the pressure on the outer ear is greater than the pressure in the middle ear, the eardrum bends inward, causing pain. If the ears are not equalized, complications such as infections or ruptures can arise.
As a rule of thumb, a swimmer should equalize his ears before he feels any pain. The pain is usually restricted to descents, and the ears naturally equalize as a swimmer rises in the water, with the extra air pressure in the middle ear diverted out through the Eustachian tube.