If you walk into a Finnish sauna and see someone rhythmically beating themselves with a bunch of leafy branches, please do not call the authorities. This person is not in distress. They are actually experiencing the peak of Finnish wellness. This bundle of joy is called a vihta or a vasta depending on exactly which part of Finland you want to start a heated argument in. To the uninitiated, it looks like a DIY gardening accident, but to a Finn, it is a dense bouquet of pure bliss and the literal scent of summer.
A proper whisk is best made from young birch branches with fresh leaves. The ideal time for this is around Midsummer because that is when the leaves are strong enough to stay on the branch rather than flying off like green confetti. You bundle a large number of thin and flexible branches into a dense bouquet and keep it in a bucket of water between rounds. Using the damp whisk in the intense heat of the sauna releases an aroma that defines the Finnish summer. While you can be experimental and use other bushes, birch remains the original choice for any purist.
Finnish immigrants in Australia even found a way to keep the tradition alive when they faced a devastating birch-less crisis. They simply looked at the local flora and chose eucalyptus branches instead. It likely smells quite nice in its own way. Whether you use the traditional birch or the Australian alternative, the process of whisking has to be experienced to be understood.
The science behind the leaves is quite impressive too. Birch leaves contain flavonoids and tannins along with saponins which act as a natural and mild soap. These aromatic compounds provide antibacterial and anti inflammatory effects that can even help soothe the itch of mosquito bites. The intention is never to cause pain. The striking can be firm to boost circulation, but it feels surprisingly good and is not masochistic at all.
However, the intensity can certainly vary depending on who is holding the branches. I once had a friend whose father was a police officer and a generally forceful guy. He whisked me so vigorously that my eyes actually started to water. He meant it in the most friendly way possible, but he certainly brought a lot of law and order into that sauna session and did not hold back.
Fresh whisk season is unfortunately short and while you can buy them dried or frozen, they are never quite as good as the fresh ones. Typically one whisk will last for one to three sauna sessions before it loses its magic. It is a small but pleasant addition to the ritual that connects the bather directly to nature.
You can find more information about the history and use of the whisk at the link below.