So I solved the mystery about the weird “ass steamers” at the gym. According to this website it’s a sort of medicinal treatment. This has been bothering me for months now, trying to figure out what exactly is going on with the occasional instance when I walk into the gym locker room to find a woman sitting on a weird bench with a hole in it, with a pot of herbs in water steaming below, a plastic tarp-like thing around her body and a shower cap on head.
“Sometimes there are alternative medicine therapies available which may look very strange to western eyes. There is a mugwort treatment therapy called SSuk Jim, in which a lady will sit on a special wooden chair with a hole on the seat. An ajuma boils a concotion of herbs in a clay pot (mugwort, green tea and some others which don’t have an English translation). The medicinal vapor comes up from the hole in the chair treating gynecological disorders such as menstrual irregularity and cystitis.”
That said, I still suspect that it may also be a facial (of sorts) for your butt.