A very good Hardanger fiddling CD that a Norwegian friend gave me is Devil's Tune, a compilation of 16 tunes from about 11 different Norwegian fiddlers. The liner notes have very good information on this fiddle tradition and the tunes.
"The four upper strings are often tuned in different ways to create a hypnotic archaic drone, which sometimes produces an altered state of awareness in fiddlers and listeners alike. In fact, a deep state of trance may occur during which the fiddler, in particular, may have visions of beings from another dimension who teach him or her new tunes. There are many stories about exhausted fiddlers who, after days of playing, have achieved a trance-like state of ecstasy. In this state they have had access to a magic doorway into a waking dream, or twilight zone, in which myth, legend, dream and reality blend into one state of consciousness. Many well-known fiddle tunes have been inspired by such a state..."
"It is said that as many as 16 different tunings exist and are used on the Hardanger fiddle today, although some are used only in specific local districts. The names given to these different tunings are often connected with colors such as "light blue", twilight grey", and half-grey". Others are named for legendary beings such as "hulder" (wood nymph) and "troll" and still other names are simply a description of how the fiddle is retuned."
The liner notes go on to talk more about the decoration of the instrument, the rhythms and other musical attributes of the tunes, and so on.
The recordings themselves span the years 1980 to 1998, and all are beautiful examples of this music. I HIGHLY recommend it if you like this style of fiddling.