Verified answer

Most information on seidr is conjectural (including that from books by neo-hippie-new-age "experts"), as it was A) secretive to begin with, and B) lost during the Christianization of Scandinavia.

About all that is really known about it is that it was considered a feminine magical practice (the male magical practice was called galdr). Not that men didn't practice it, but that it was a woman's art. Thus some people believe it involved sexual rituals.

But really, what it was and how it worked, have been lost to history. All people do now is make guesses - some educated, some very uneducated.

seidr is a kind of trance state that causes spontaneous movement or shaking--it is a kind of energy raising or shamanic meditiative state. It is seen in many esoteric cultures and is called different names. To learn more about seidr and its use in magick, look up the author Jan Fries. He is a post-modern occult who has written a book or two about seidr and magick.