To the untrained eye, many dressage movements can look the same. When you’re watching experienced riders, you may not even notice the different aides they use in order to push their horse into a new gait or into a new movement. For movements like the leg-yield and shoulder in, you even see horses performing similar steps even though what they’re performing is very different.
A leg-yield is performed when a horse is straight and moves away from the rider’s inside leg by crossing their hind legs in front of one another. When done correctly, the horse will move diagonally “on two tracks forward and sideways”. According to Gerhard Plotz’s “Lessons in Leg Yielding”, the only bend in this movement comes from the “slight flexion at the poll away from the direction of the movement, so that the rider is just able to see the horse’s inside eye and nostril.” When the movement is performed, the horse and rider are moving diagonally — but your horse’s shoulders should not be leading your direction. This is where the two movements begin to differ.
With a shoulder in, according to USDF Directives, your horse’s inside front leg passes and crosses in front of the outside front leg. At the same time, the inside hind leg is stepping forward into the same spot the outside hind leg was just in. The horse are rider are still moving in a straight line, but the shoulders are angled at approximately 30 degrees. Instead of using the inside leg as your cue, you use the position of your body. Emile Faurie explains that the rider should be sitting “central in the saddle. [You should] position your own shoulders to be parallel with the bend of the horse’s shoulders.” The inside rein, however, is not what’s giving the cue to bend: it must come from the horse’s flexion.
The difference between these two lateral movements when explained on paper doesn’t seem that big. When they’re witnessed in person, you have to be extremely careful to pay attention the way the rider moves their upper body independently from their lower body. But they’re movements that separate different levels of riders, and horses, for a reason. The higher in dressage levels you go, the more technical movements become. This is why it’s so important for the dressage rider to develop the ability for their hands to be independent of their legs and even their arms.