The fiddles are played the same as a regular fiddle, the instrument of course. The drone or sympathetic strings run from the pegs UNDERNEATH the fingerboard, down to the bridge, underneath the regular strings, so they can't be fingered while playing. You might even be able to get a traditional Norwegian instrument by Googling "Husfliden", and searching for one. Husfliden is the Norwegian national crafts sales company, a bit like a central Cecil Sharpe House, with stores in most cities selling native crafts of their area as well as the whole of Norway.
I have never played a Hardanger Fiddle, but I've seen them around Norway when I was over there with the forces. There was a Norwegian Air Force pilot who was famous around their military for having made and learning to play a Hardanger Fiddle. I think they are an interesting instrument, for the use of the drone strings, but since I don't play fiddle at all, would be a complete waste for me. Not that this has ever stopped me in the past.