Masking activates your body's self-cleansing mechanism (a.k.a., lymphatic system). For the mask to work properly, you must apply it in a thick layer, which prevents any exchange between the skin and the environment. This technique creates a suffocation effect, causing the nerve endings to send a message to the brain that says, help. The brain responds by directing the lymphatic system to send excess blood serum to the skin. Blood picks up excess moisture within the tissue and carries it to the skin.
This technique simultaneously stimulates the eccrine gland to fight the invading mask by excreting accumulated waste products. Basically, masking stimulates blood flow and brings oxygen, nutrients, and hydration to your skin and carries away the waste products.