If you watched American swimmer Michael Phelps take the starting block in Sunday’s men’s 4×100 freestyle relay, you may have noticed several circular marks on his torso.

Other athletes have posted photos on social media of themselves with similar markings.

Turns out the circles are caused by a technique called cupping, which entails having small cups placed on their skin. A pump sucks the air out creating enough suction to pull the skin into the cup, causing the markings.

Some athletes swear by cupping as a good way to recover quickly after training. Others say it increases the blood flow to sore muscles, easing the pain.

Cupping dates back to at least ancient Egypt, according to the medical website WebMD.It has also been used in Chinese and Middle Eastern medicine.

The marks can last up to two weeks.

Cupping appeared to have worked for Phelps in the relay race as he swam an impressive second leg, vaulting the U.S. team to the gold.