When Michael Phelps dives into the Olympic pool, dark circles are visible on his shoulders and his back.

The swimming star is obviously into cupping.

David Arquette once jokingly referred to the marks as “giant hickeys.”

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An ancient Chinese method of healing, cupping is done to enhance blood flow and provide relief from muscle pain and stress. The way it works is this: Glass cups that look like small fish bowls are heated and placed on the skin, creating a vacuum, drawing the skin up into the glass. When they are taken off, marks are visible.

It’s working for Phelps, who snagged his 19th Olympic gold medal on Sunday, part of the victorious Team USA’s 4×100-meter freestyle relay.

“Some really benefit from the treatment,” Gustavus Adolphus College coach Jon Carlson told Swimming World in a 2015 article. “Others have tried it, and have moved on to other forms of treatment.”

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It’s not just for swimmers, though — Hollywood stars have long been into the trend.

Three years ago, Jennifer Aniston’s cupping marks could be seen as she wore a strapless outfit on a red carpet premiere. Gwyneth Paltrow also showed off bruise marks after she tried the therapy, and Jessica Simpson, way back in 2010, raved about the technique. David Arquette, after ex-wife Courteney Cox underwent treatment, once jokingly referred to the marks as “giant hickeys.”

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Cupping is a form of acupuncture that focuses on the movement of blood, energy or “qi,” and body fluids that circulate around the body’s tissues. When blood isn’t flowing, stagnation can happen. Cupping is then employed to stimulate the area. It is not just for for muscle relief — it may clear congestion in the chest, which can occur with colds and flu.

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It’s not a Food and Drug Administration-approved treatment but it’s been around for, oh, a few centuries. Professor Edzard Ernst from the department of complementary medicine at the University of Exeter says the practice can cause burns, but otherwise is relatively safe.

As for whether it works, he told the BBC in 20014: “There is no evidence for its efficacy. It has not been submitted to clinical trials, but there have certainly been satisfied customers for 3,000 years.”

Alex Naddour, on the U.S. men’s gymnastics team, is also a proponent. After trying massages and getting cortisone shots, he turned to cupping, even buying the equipment for $15 on Amazon and asking teammates to help him.

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“You’re like, ‘OK, I’m sore here,'” team captain Chris Brooks told USA Today. “Throw a cup on, and your roommate will help you or you can do it yourself.”

Naddour says he prefers it to having a massage because it helps pull the tendons and muscles up, instead of having them pushed down.

“That’s been the secret that I have had through this year that keeps me healthy,” Naddour said. “It’s been better than any money I’ve spent on anything else.”