The grass – which contains such medicinal herbs as Thyme, Arnica, Silverweed, and Gentian – is harvested only from unfertilized meadows, and always either early in the morning or late in the evening, so that all of the essential substances are preserved. And then it's dried as quickly as possible.
Body warmth causes the hay to release the numerous active ingredients (including essential oils), which are absorbed by the bather via his skin and respiration. But for this to happen, it has to first be moistened.