Basically, cupping involves a cup attached to a pump. Athletes put the cup on their skin and create suction with the pump. Some say the technique increases blood flow and helps a person’s sore muscles heal.
According to WebMD, cupping therapy “dates back to ancient Egyptian, Chinese, and Middle Eastern cultures.”
Phelps apparently uses a version of the therapy that involves heat to create suction (it was even featured in his Under Armor ad announcing his return to competitive swimming).