Michael Phelps and the Olympic swimmers arrived in prime time Sunday night, and there was something and unmistakable on their bodies: round circles.

They weren’t the result of a tattooing misadventure or a secret symbol known only to members of the swim team. Either of those would have made a great story. The circles came from cupping, a technique used by trainers who attach suction cups to pull blood to sore and injured areas to speed healing. A recent Under Armour video shows Phelps receiving the treatment, as he has done for years. He also posted a photo on Instagram last year, telling fellow Olympian swimmer Allison Schmitt, “Thanks for my cupping today!”

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

At about the 1:30 mark of Under Armour’s video, you’ll see what cupping looks like. (Or you pay attention to all things Gwyneth Paltrow, you’ll know.) If you’ve ever had it done (I have, but not because Gwyneth recommended it), you’ll find it relaxing and it does seem to make your muscles feel better — perhaps because it stretches tendons and muscles differently than massage.

The cups, which create suction with either heat or little pumps, aren’t attached for long, and the discoloration is the result of broken capillaries that occur as the skin is pulled up into the cup.

Does it work? Experts are divided on the matter. But if you believe it works, you’re likely to perceive that it’s beneficial. I haven’t had it done lately, but I’m an Olympic blogger, not an Olympic athlete. Would I have it done again? Sure.

Swimmers aren’t the only ones who are trying the technique. Alexander Naddour, a Team USA gymnast, sported circle bruises.  Alexander Naddour, a do-it-yourself cupper thanks to a kit he bought for $15 on Amazon, was sporting the purple dots during competition Saturday in Rio. “That’s been the secret that I have had through this year that keeps me healthy,” Naddour told USA Today. “It’s been better than any money I’ve spent on anything else.”