I've been a huge fan of seances & "spiritualism" since before I knew what those words meant. When I was 10, I tried to contact the recently deceased Heather O'Rourke via Ouija Board, and I have always loved seance scenes in the movies.
This trick would have been great to witness back in the early 1900's, when we didn't have a ton of technology and our minds would have been more susceptible to this kind of thing- and I was excited to find out how the trick actually works via The Life and Mysteries of Dr. Q @ google books.
I thought that "Dark Seance" was meant as an Occult selling point, but the trick is actually performed in the dark. This is so the audience- which is sitting around you in a circle- cannot see that the trumpet you placed between your legs has a long black hose coming from it to your mouth, where you can voice the "secrets of the dead."
Kind of hokey when you know how it works, and you would really have to practice to make sure they can't hear you mumbling into the tube, but it would be fun to see an experienced Magician perform it.