He describes the movement while tracking left, "start the long side in shoulder-in left. After a couple of steps of shoulder-in, straighten the horse as though to ride on the diagonal. Proceed straight on the diagonal for two steps only, then re-position the horse in shoulder-in and ride shoulder-in position back to the wall. When arriving on the track with the outside hind leg, proceed again straight on the diagonal for two steps, and so forth down the track."
The exercise he says can be ridden in all three gaits and I have found in canter for this to be something akin to riding between shoulder-in and "plie" as Charles de Kunffy describes it. Overall, the process honestly puts the rider and horse onto the outside rein while teaching the rider to subtly control the horse by being acutely aware of "both sides". Key points are to remember not to lose the angle or have the haunches swinging away. We straighten a horse always by positioning the shoulders ahead of the haunches in order to follow the classical maxim of "ride the horse forward and straighten him." As with all exercises of course it cannot be attempted without the proper preparation and so the horse should already know the shoulder-in.
I find this exercise also particularly helpful with horses who are a bit tentative to go to the outside rein. The almost leg-yield like moment of SI headed back to the wall gives the horse a place to go and can help them build confidence in stepping with the inside hind towards the outside rein which is in this case traveling a little away from the leg as the line of travel is changing.
Horses on the other hand who like to "blast through it" should be ridden carefully in this exercise with great thoughtful control of the outside aids so that they don't 'bulge' through the shoulder back to the wall. I like making changes from shoulder-in to renvers to shoulder-in with such horses.
For more of Walter Zettl try his other book The Circle of Trust: Reflections on the Essence of Horses and Horsemanship
or his wonderful DVD's A Matter of Trust: A Harmonious Partnership Between Horse & Rider Vol. 1 Walter Zettl 2 DVD Set
