Cupping is a vacuum type massage technique in which suction is applied to the skin using plastic cups to any area on the body. This technique increases blood flow to the area and releases any tightness in muscles, joints, and fascia. Cupping reduces tension on the skin and tissues below to loosen adhesions, lift up any restrictions in connective tissue, and release tight muscles. It can be used to move inflammation to the surface of the skin to release and drain extra fluid and toxins to the lymphatic system and therefore drain out of the body. 
Cupping is beneficial in relieving tightness to tissues by relaxing the muscle and allowing the fibers to separate. This allows increases in range of motion as well as flexibility of the area. Cupping is also useful to treat high tone areas when muscles tighten up and limit an individual’s movement. An increase in temperature and blood circulation allows a breakdown in adhesions and relaxes surrounding tissues.
 

Source: www.physio.co.uk