I have kinda been out of circulation for a few days. Let me address some of the replies.
First, ground work is not the most effective way to address getting a horse to move its shoulders. But then, all of my horses DO move their shoulders when I am on the ground. I consider it a VERY BIG PART of having good manners. I step into a horse's shoulder and smooch, he had better move his shoulder (or hip or whole body) but it is so much a part of all my handling, I just never spend any time concentrating on it.
First, I need to go back to basic training knowledge that most people do not ever even think about. There are two very different 'sets of aids'.
The lateral aids -- This is when the rider uses a dominant leg and a dominant rein on the same side. This would be what a 'leg yielding' exercise is and would be what most people use when they teach a turn on the forehand. The rider's other leg and rein can be a 'steadying' influence, but the dominant aids are on the same side. These are generally very basic beginning maneuvers used on green horses and used most by green riders.
The diagonal aids -- This is when the rider's dominant aids are on opposite sides -- like using a dominant right rein and a dominant left leg at the same time. [Often, the 'outside leg' is moved slightly back.] These are the aids used for more advanced maneuvers and often require much more rider skill and experience to get good results. These are the aids used to teach spins, roll-backs, 'proper' half passes, good side-passes, good lead departures, flying lead changes, and the list of advanced maneuvers goes on and on. The proper execution of these maneuvers requires that the rider can move the horse's head and shoulders to the inside of the track with a dominant inside rein, can keep the horses ribs out, can keep the horse's inside shoulder 'up' and can bring the horses hip to the inside by using a dominant outside leg -- all at the same time . The rider's inside leg keeps the shoulders up and the ribs out; the outside rein 'steadies' the horse; but the outside leg is the dominant leg. This is advanced training with an advanced rider.
Now, let me clarify what happens when most people teach a horse to disengage its hind quarters and teach it to do a turn on the forehand:
The inside rein 'holds' the horse from going forward and keeps the horse from moving while the inside leg pushes the horse's hip to the outside. It is really simple. Only requires the horse to learn to move from simple pressure. This is why it is the first thing a green horse is taught -- right along with doing forward. The head and neck are bent towards the dominant rein.
But, like I have warned so many times, we often think we are teaching one thing while the horse is learning something else. It is really easy for us to think that the inside leg pressure is making the horse move -- and it may be helping. What the horse may be learning is that when the rider pulls on the right rein, the horse should move its hip to the left in response.
Now, let's say, you want to open a gate. It is very easy to get a horse to bring its hip next to the gate by taking its head the opposite direction. Just like the turn on the forehand, this can be done with a more dominant outside rein and a dominant outside leg. The rider is grinning from ear to ear because the horse has moved its hip away from the rider's leg and he can open the gate. Actually, the horse may be just moving its hip to the left because the rider has taken its head to the right. At first, it may look and feel like these two things are the same, but nothing could be farther from the reality of what is happening.
In reality -- what the horse MAY be learning is that the pull on the outside rein is the aid that is the 'cue' to move his hip the opposite direction. If you take hold of the right rein and the learned response is for the horse to move its hip to the left, what do you think he is going to do when you take hold of the right rein and really want him to move his shoulders to the right? Do you see what I am trying to say here? If you have taught him that taking his head to the right is the cue to step his butt to the left, how can you expect a different result when you really want him to move his shoulder to the right to turn with a cow?
If you want a horse to work cattle, you MUST teach him to hold his ground behind. That means that you should be able to take his head toward the cow (look at the cow with both eyes) and he should keep his butt in place -- not 'flop' it out away from the cow. He cannot turn (or roll-back) if his arse is literally out in left field and not under him to turn on. We use a disengagement to take a horse's power away from him -- like when we think he may want to buck. He also has no power to turn around when his hip is 'out there'.
This is why I never teach a disengagement of a horse's hind quarters as a routine part of teaching a 'one rein stop'. Yes, I teach a horse to disengage its hind quarters but seldom practice it or practice turns on the forehand. It is a lot more difficult to get a horse to consistently and lightly move its shoulders when asked. Since moving the shoulders and 'holding' their hind quarters in place requires using BOTH the inside rein and the outside leg, it can be done best while mounted. It can also be done easiest when the horse does not automatically move its butt away from the dominant rein but towards it.
What I prefer to see in side-passing and in leg yielding exercises (and always strive for myself) on anything but the very 'greenest' of horses is a steady head with little or no pressure on the lateral rein, a straight or nearly straight body from nose to tail, and the horse moving lightly over to the gate or side-passing for whatever reason.
I hope this makes sense.