Like Clockwork
As you keep working on this exercise, your horse will begin to walk diagonally away from you when you ask. It helps at this point to visualize the hands of a clock. Start at 12 o'clock, which is straight, forward movement. Move to 6 o'clock, which is backing up. Just ask him for a slight angle at first, perhaps at 1 o'clock (or 11 o'clock if you're on his right side at this point).
You can have a lot of fun with the numbers on a clock with this exercise, starting each number on the ground and then working on it in the saddle once your horse seems relaxed and confident. Remember that this is physically demanding work for him, so work in small increments, rewarding him with instant release every time he tries to cooperate.
As his skill level improves, he'll need less and less neck bend to shift over when you ask. When you're riding, you can control this with the opposite rein. Eventually, you want him to stay nearly straight and lift his shoulder with a pretty little neck bend in the direction you're asking him to go, but that's a ways down the road.
At 2 o'clock (or 10 o'clock), you're doing a very nice diagonal movement, with your horse stepping deeply under his body. This is where the weightlifting comes in, so take it in small stages. Once he's mastered this on the ground, showing prompt, relaxed responses, he'll be ready to work on it with you in the saddle.
Saddle Work
When you're mounted, repeat the process. Walk your horse briskly forward. Take the slack out of your left rein, asking his nose to come toward that spot on the base of his neck. Don't let him slow down, but keep the pressure until he shifts his weight to the right, then immediately release and keep walking straight. Repeat, and switch sides.
If your work on the ground has been solid, your horse should catch on pretty fast when you're up top. If you hit a snag, go back on the ground and review that work.
Take however much time your horse requires to get relaxed and confident about this maneuver, because now it gets fun as you begin to dance with your horse.
Giving your horse frequent praise and breaks, take him diagonally to the right for a few strides, then diagonally to the left for a few strides, then to the right, then to the left. The distance is not as important as the frequent changes in direction.
In the early stages, especially, he may look like a drunk coming out of a bar late at night, but as his strength and agility improve, you'll find he's getting softer in the way he carries the bit, is more balanced underneath you, and that he's really enjoying this, as well. Once he has that figured out and is physically comfortable, do these "o'clocks" at the trot.
At 3 o'clock (or 9 o'clock), you're doing a full sidepass. Your horse must stretch muscles he hasn't used, so again, take it slowly.
When you're at 4 or 5 o'clock (or 7 or 8 o'clock), you're asking the most of your horse, so try for only 1 or 2 steps at most as you begin this one. You're now asking him to go diagonally backward, and you'll see his "pushing away leg" deepen and really work.
The Dance
If you've done your homework on the ground and kept your horse relaxed, you should now be able to move his shoulders anywhere you want them while keeping him in excellent balance. Spins, diagonals, lead changes, stops, or whatever else you want to teach him will be easier with the partnership you've now established.
Take your time, have fun, keep building toward perfection, and you can do darned near anything with your horse that you want to do.
Go dance!
