I've read about these strange people behind this mind-altering music. According to several reliable sources, at summertime it is highly advisable not to enter the Finnish woods.
At nighttime there is a risk to encounter these bioluminent druids, neo-paganists etc.
The seemingly uncontrollable jolting and swaying in a ritualistic manner is sometimes used to summon "Hyvis". Unfortunately I couldn't find anything more about this deity. Be cautious!