Guessing it’s this:

Birch branch beating

Among the sauna paraphernalia sold at stores you may be bemused to see thin leafy branches from birch trees. Well, it’s actually common practice to use soft birch branches to beat your skin (or others’) in the sauna. It may sound strange, but the beatings are believed to improve circulation and, as a bonus, help relieve mosquito bites.

https://theculturetrip.com/europe/finland/articles/the-most-bizarre-finnish-habits-in-saunas/

I think is called viht / vihad but not sure. nope, I was wrong. see Fisuxcel’s comment below

Btw, the hanging snacks to cook while you steam thing mentioned in the article sounds freaking awesome… But Imma fatass and love food so I might be biased there. And I can feel the gay jokes from “hanging sausages” about to roll in.