Hard to believe someone would think they could fool those two with that old one, but all you have to do when someone is tying you up is tighten up your hands and in this case legs as you're being tied up. Then you can relax and easily escape from the rope. Apparently, they can slip their hands back into the rope and make it appear that they're still tied up; but I've never seen that done before. One of the Hardy Boys books gave that one away, but I don't remember which one.
Take care and God bless