Cupping pulls blood to a region to stimulate healing. It is effective at stretching tight fascia and muscles. Cupping helps the tissue develop new blood flow, also causing anti-inflammatory chemicals in the body. Cupping is generally painless and many say is more effective than massage for muscle problems.
The two theories of cupping that I have been taught are:
The vacuum in the cup brings blood and lymph to the area, promoting circulation and healing. In addition it will help break adhesions between the skin and underlying connective tissues, allowing for increased fascia and muscle movement.
All pain and disease in the body is caused by stagnant chi. The reverse pressure of the cup pulls blockages out of the chi meridians, thus allowing for free flow of chi, and a return to healing and normal chi flow.