Shoulder in is one of the most useful exercises that we have available in Dressage. Done correctly, it will engage and increase the carrying power of the horse's inside hind leg. Which will bring the horse into better balance. It also increases their lateral suppleness, teaches horse and rider the concept of riding from the inside leg into the outside hand, and gives the rider the tools to improve the horse's general straightness.
To obtain these benefits, shoulder in must be done correctly. Much of the time riders think they are doing shoulder in, but have little bend in the horse's body, turning it into a leg yield instead. And while leg yielding is a useful exercise for improving obedience and understanding of lateral aids, and increasing lateral suppleness, it does not have the engaging and collecting effect that shoulder in does. Here are two pictures which clearly show the difference between shoulder in and tail to the wall leg yield: (Click on Article Title above (in blue) to read full article.)