If you caught Michael Phelps winning his 19th gold medal during the 4x100 meter freestyle relay Sunday, you might have wondered why the highly decorated Olympian was sporting circular bruises on his back. Do Olympic pools have predatory octopi?

Fortunately, no. The bruises are from a holistic practice called cupping, which uses small glass suction cups that are placed on targeted areas in an attempt to release tension from the muscles.

Some athletes use an air pump attached to the cups, which creates a pressurized site that pulls on sore and tender muscles and tendons. Other, like Phelps, utilize a heat method, where a flammable liquid-soaked cotton ball is set on fire in the cup before being doused and applied to the skin. (If Phelps is ever reported to have spontaneously combusted, this would be a plausible explanation.)

In events where success or failure can be measured in tiny increments, it’s not surprising athletes will turn to any potential advantage. But does cupping actually work? A hallmark of traditional Chinese medicine, its efficacy is still unclear. Advocates like Gustavus Adolphus College swimming coach Jon Carlson say the “de-compressive” nature of cupping—the suction lifts skin off muscle and bone, which lets blood vessels expand and increase blood flow for rapid healing—makes it beneficial. Because the body believes it might be hurt, it rallies to attend to the problem.

It’s a plausible theory, but with pain being a highly subjective area of research, few reliable studies exist: It’s hard to perform a “blind” therapy when hot glass is being vacuumed to your skin.

Still, you’ll probably see the marks on other athletes: Gymnasts are also fans of the technique, and it’s even available in the official Olympic Training Center in Colorado Springs, Colorado.