In cupping , through either heat or suction, the skin is gently drawn upwards by creating a vacuum in a cup over the target area of the skin. That suction pulls the skin up into the cup, breaking capillaries and causing blood to pool and stagnate, creating a bruise. The cup stays in place for three to fifteen minutes. It is believed by some to help treat pain, deep scar tissues in the muscles and connective tissue, muscle knots, and swelling. These people believe the process helps muscles to heal more quickly by encouraging blood flow.
Broadly speaking there are two types of cupping: dry cupping and bleeding and/or wet cupping (controlled bleeding where light scratches are made in the skin before the hot cup is applied), with wet cupping being more common as it is mentioned in the Koran and is popular in Muslim cultures.
Cupping takes a big bashing from mainstream medicine, which argues that cupping creates a bruise, which is a blood clot and definitionally not flowing. But people who’ve tried it, say it works for them. And Phelps has a shedload of medals to back up his endorsement of the practice.