Shoulder-in is the father of the advanced lateral dressage movements. It does many wonderful things for your horse. Here are just some of them:
Shoulder-in is a suppling exercise because it stretches and loosens the muscles and ligaments of the inside shoulder and forearm. During shoulder-in, your horse passes his inside foreleg in front of his outside foreleg. This motion increases his ability to move his forearm gymnastically in other movements.
It’s also a straightening exercise because you should always straighten your horse by bringing his forehand in front of his hindquarters. Never try to straighten him by leg yielding his hindquarters out behind his shoulders.
Shoulder-in is also a collecting exercise. It increases your horse’s self-carriage because he lowers his inside hip with each step. As a result, his center of gravity shifts back toward his hind legs. His hindquarters carry more weight and his front end elevates. Read more> http://www.horsesinthesouth.com/article/article_detail.aspx?id=10240