According to the back of this Postcrossing card from Taru, “the bath whisk of birch twigs must be soft and warm.”  See, honey?  This postcard is educational!  Actually, the missus had a few comments on this young lady’s physique, but she has nothing to worry about.

The description is also published in Finnish — “Saunavihdan haudutus” — which, as I recall, is the spell that Harry and Ron tried to use in Book 5 to lure Hermione into the steam room at Hogwarts.