helmet fire,
Early H-3's had some "tail shake" due to rotorhead wash going into the tail, solved with a carefully shape main rotor pylon collar (the horse collar). In my first days in the Pilot's Office, I had been told this was the "shuffle" and that it was not intrinsic to any other Sikorskys. I have always thought the "Sikorsky Shuffle" was more of a conveniently aliterative name than an actual condition.