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Have you been wondering why swimmer Michael Phelps and other Olympians are sporting deep-purple circles on their limbs and midsections?

While it may look like the athletes have been in a bar fight, the telltale purple dots actually are signs of “cupping,” an ancient Chinese healing practice that is experiencing an Olympic moment.

In cupping, practitioners of the healing technique — or sometimes the athletes themselves — place specialized, round circular cups on the skin. Then they use either heat or an air pump to create suction between the cup and the skin, pulling the skin slightly up and away from the underlying muscles.

The suction typically only lasts for a few minutes, but it’s enough time to cause the capillaries just beneath the surface to rupture, creating the circular, photogenic bruises that have been so visible on Mr. Phelps as well as members of the U.S. men’s gymnastics team. If the bruising effect looks oddly familiar, it’s because it’s the same thing that happens when someone sucks on your neck and leaves a hickey.

Thanks @arschmitty for my cupping today!!! #mpswim #mp @chasekalisz

A photo posted by Michael Phelps (@m_phelps00) on Sep 10, 2015 at 12:29pm PDT

Physiologically, cupping is thought to draw blood to the affected area, reducing soreness and speeding healing of overworked muscles. Athletes who use it swear by it, saying it keeps them injury free and speeds recovery. Mr. Phelps, whose shoulders were dotted with the purple marks as he powered his 4×100 freestyle relay team to a gold medal Sunday, featured a cupping treatment in a recent Under Armour video. He also posted an Instagram photo showing himself stretched on a table as his teammate, fellow Olympian swimmer Allison Schmitt, placed several pressurized cups along the back of his thighs. “Thanks for my cupping today!” he wrote.

While there’s no question athletes and many coaches and trainers believe in the treatment, there’s not much science to determine whether cupping offers a real physiological benefit or whether the athletes simply are enjoying a placebo effect.

One 2012 study of 61 people with chronic neck pain compared cupping to a technique called progressive muscle relaxation, or P.M.R., during which a patient deliberately tenses his muscles and then focuses on relaxing them. Half the patients used cupping while the other half used P.M.R. Both patient groups reported similar reductions in pain after 12 weeks of treatment. Notably, the patients who had used cupping scored higher on measurements of well-being and felt less pain when pressure was applied to the area. Even so, the researchers noted that more study is needed to determine the potential benefits of cupping.

Another experiment involving 40 patients who suffered from knee arthritis found that people who underwent cupping reported less pain after four months compared to arthritis sufferers in a control group who were not treated. But the cupped group knew they were being treated — it’s not easy to blind people about whether a suction cup is being attached to their leg or not — and so the benefits might have been due primarily to a placebo effect.

Still, a placebo effect can be beneficial, and for athletes at the Olympic level any legal edge, however tenuous, may be worth a few eye-catching bruises.

A few years ago Denver Broncos player DeMarcus Ware posted a photo on Instagram showing his back covered with 19 clear cups as a therapist held a flame used to heat the cup before placing it on the skin. Celebrities including Jennifer Aniston and Gwyneth Paltrow have also been photographed with cupping marks on their skin.

Last year, Swimming World magazine noted that some college programs had begun using cupping therapy as well as former Olympian Natalie Coughlin, who has posted a number of photos of herself undergoing the treatment.

U.S. gymnast Alexander Naddour was sporting the purple dots during the men’s qualifying rounds on Saturday in Rio. He told USA Today that he bought a do-it-yourself cupping kit from Amazon. “That’s been the secret that I have had through this year that keeps me healthy,” Mr. Naddour told USA Today. “It’s been better than any money I’ve spent on anything else.”