Cupping is a therapy that uses hot cups to loosen tight muscles, and some Olympic athletes swear by it.

American swimmer Michael Phelps is sporting the tell-tale round bruises associated with the practice. It can be done with a suction machine, but some therapists prefer the traditional flame cupping. The theory is that the suction increases blood flow to an afflicted area.

“Tissue quality is a huge part of it, so a lot of times, body composition, if we are a really lean person, the cups will get down into deeper tissues a lot more efficiently,” said Jeff Burgess, registered massage therapist. “If we are somebody who has excess stored energy on our body, then it’s not going to be as effective. So it’s not meant for everybody but it definitely has a benefit for most athletes.”

But does the treatment really work? Several studies report that patients say it does, but the research gets some criticism for lack of effective control groups, and there are concerns the studies may be biased. But for those who use it, cupping is believing.

“Basically, it’s another tool in the toolbox,” said Murray Heber, registered massage therapist. “It’s how you apply that tool and how you use it, so that’s the magic thing with cupping or these different modalities, that used the right way at the right time, it can have excellent effects.”

It seems to be working for Michael Phelps, as the Olympian’s gold medal count hit 21 in Rio on Tuesday.