When your sinuses are congested, air cannot freely pass in and out of the sinus cavity. This prevents equalization of the sinus, and causes a painful squeeze during descent. If you feel pressure anywhere behind your face during descent, your sinuses aren't equalizing, and you should abort the dive.

As a sinus squeezes, the soft tissues that line the sinus are stretched inward. This causes the blood vessels to rupture and fill the sinus with blood. Then, as you ascend, the remaining air inside the sinus expands. Initially, the blood inside the sinus prevents the air from returning to its original volume. This causes painful pressure against the lining of the sinus. Eventually, the pressure frees the congestion and forces the blood and mucous out of the sinus and into your nasal passage.

A sinus squeeze results in a bloody nose, but this does not mean you must suspect a sinus squeeze whenever you have a bloody nose. Many people suffer from bloody noses as a result of pressure changes, so don't be alarmed unless it's accompanied by pain in the sinus area.