Eagle-eyed Olympic observers probably noticed on Saturday those small, circular marks on the backs and shoulders of several Team USA athletes, including members of the men’s gymnastics team and even all-world swimmer Michael Phelps.

Those circles are the end product of a healing technique called cupping.

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).

One downside of the therapy, however, are the bruises. According to an article in The State News about the Michigan State gymnastics team’s use of cupping therapy, the marks can last up to two weeks.

Team USA gymnast Alex Naddour recently posted an Instagram picture of himself with a cupping mark clearly in view.

Former U.S. Olympic swimmer Natalie Coughlin has shown off her own cupping bruises in the past on Instagram, drawing some quizzical responses from her followers.

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The same kind of response is happening during the Olympics as many viewers see the marks for the first time.

Some, however, are more skeptical of the benefits than others.