The shoulder-in is a lateral movement that promotes control of the horse's inside hind leg, which is critical to the correct execution of a number of upper-level movements. The shoulder-in position causes the horse to stretch its inside hind leg a little farther sideways than the normal bend, which results in the horse lowering its inside hip in collection. This can be very useful on a strong, stiff horse because it prevents the animal from bracing with its inside hind foot, using it as a base from which to pull against the reins.
The shoulder-in is executed at the walk or sitting trot on the long side of the arena. In a left shoulder-in position, the horse is moving on three tracks, with its left fore on one track, its left hind and right fore on a second track, and its right hind on a third track. You should concentrate on the centerline, formed by the left hind and right fore, and drive the horse's feet forward along that line to maintain the correct position. You can best do this by looking down the line and feeling the horse's left hind leg reach under your seat with each step.
To perform the left shoulder-in when moving counterclockwise, first balance the horse with half-halts, then bring both of your hands slightly to the left, moving the horse's forehand in that direction until the animal is at a 30-degree angle from the rail. The moment you displace the shoulders, press with your left leg in an at-the-girth position to begin lateral movement. (The at-the-girth position of your inside leg, coordinated with an inside indirect rein, causes the horse to be bent uniformly from head to tail. Note the difference between this and the behind-the-girth position of the inside leg during the leg-yield, which causes the horse to remain straight from withers to tail.)
Throughout the movement, the driving aid is predominantly your left leg, although the right leg helps in maintaining impulsion. Your left leg and hand sustain a slight bend to the left throughout the movement; your right hand controls the pace and prevents the horse from "popping its shoulder" to the right; and your right leg prevents the haunches from swinging toward the rail, an error which would result in the horse's body becoming too straight from withers to tail so that the animal's fee would be tracking a leg-yield pattern.
For the left shoulder-in, then, move the forehand away from the track by bringing both hands slightly to the left, then sustain the movement with the following aids:
Since all lateral movements tend to slow down a horse, you should follow them with straightforward work that encourages long steps and an energetic tempo.