Hi and a big thanks to everyone posting in this thread!
This was a new exercise for me and one that has become absolutely essential.
Here's the progression that I use:
I use a cordeo strictly for "transfer weight back" during this training - no backing up off the neck strap. to lessen the confusion of the signals and the frustration of the horse that follows confusion...
I also use one place to practice at the beginning - and only do the school halt there (and only go there for the school halt)
- I use a corner
*teach that light taps on hind leg (I use just above the heels) mean: move hind leg forward. - this is the introduction to the whip holding the hind legs - preventing the horse from backing
*apply backwards/upwards pressure on the cordeo - until the horse shifts back (leans) - this is what happens before he backs up, the key now is to release/praise BEFORE the feet move (especially the hinds). If the hind feet move, I use the whiptap to reposition them and begin again.
once the horse understands (proven by reaction to a very light pressure every time)
* increase the weight shift. This is bound to trigger the back up at some point - that's ok, I take the opportunity to re-inforce the whip as a "holding in place" aid for the hind legs. Again, the timing of the release/reward is crucial. I watch only the haunches of the horse. At this point I don't care what he does with his neck/head/front legs. I look for the transfer of weight backwards onto stationary hind legs. That's it. Some horses do this increased demand by throwing their heads up. OK with me.
once the horse understands
* begin to ask for the joints of the hind end to flex (LS/hip/stifle/hock). At the "exaggerated lean back", either lightly tap the tail head or point of croup (experiment). the key here is surprise I think. Not to hurt the horse, but as a surprise touch on this area triggers "a tail tuck" kind of surprise. The touch has to be light, not offensive. Once again release/reward timing is key. I am still only watching the haunches, don't care what the front end does. There's bound to be some excitement with some horses now. Use it for good! They will flex their haunches. Now I find it's ok to give up on some previous "rules" for a little while. I don't mind if the horse steps back, or even jumps forward during this trial and error stage of trying to figure out what I am after. Just reward any flex of the haunches. Then slowly return to all of the above criteria. This work can be very taxing for weak horses, so use your judgement wisely... sometimes a bend in my knees/pelvis with a vertical upper body helps as a visual cue at this stage.
once the horse understands
*now it's time to address the neck/head. I begin to watch the whole horse and transfer the school halt cue to a noseband. This begins in our "school halt corner". I will ask once on the cordeo, then use light backwards pressure on the horse's head with the whip behind the hind legs. I go back to rewarding the smallest try (just like in the beginning). As some horses have learned to use the throw of the neck upward to transfer the weight backwards, it's sometimes difficult for them to now keep their head/neck steady (I will hold the rope fairly close to the horse's head to gently guide the head position). I am very gentle now and take lots of time with this step. As this is the "polishing" work, the finishing touch
With some horses, the neck needs to be addressed earlier, with some not at all. I find it goes with their general way of being: The "flightier, hollow" horses will almost always throw the neck to shift the weight, the more round, strong horses lift their poll and arch the necks into the school halt...
I haven't really played yet with asking for engagement/flexion of haunches with these cues in movement, but I am excited about the possibilities!