That's very interesting.. I like hearing and learning about how things are what they are, especially to do with older methods of ranching and whatnot.
Maybe practical for that purpose but not for 99% of what an every day horse person needs to do with their horse.
For instance, a tie down roper wants their horse to back until the pressure is tight on the rope. That would be highly impractical and even dangerous in other areas of horse handling.
I step into my horse, he backs. If I need him to back (such as out of the trailer or tie stall), I simply say 'back'.
Though I've spent all this time teaching my horse to stand still and trust the handler and those around him.. I don't want him mis-interpreting a shake of the rope to back. I prefer to voice it or use my body language.
Like I said in another thread, a parelli handler was trying to teach my horse to back (he was just showing me how any horse is trainable to the methods) and my horse got very frightened and very confused then frustrated and he backed, but it was more like a shuffle where he was leaning on his hinds rather than stepping. I don't dislike the shaking of the rope, I just didn't find it very helpful.
Maybe one day we'll try it again, but right now I have no need for it. I'm very open to trying new things with my horse :)