Acquired these from an old magic shop buy-out. They are new old stock and come with their original clear case but no written instructions are included. This is the way we received them. Once you have the trick in your hands it will become very obvious how to perform this self-working effect.
Pull any string. As one gets longer the other shortens. You can show that the long string is free from the other as you pull it back and forth through the stick and the short string does not move. But as soon as you pull on the short string the long string shortens. Very confusing to the onlookers.