When ridden correctly your hips control the horses back end, your legs control the bend in the barrel (and how forward the horse is using their hind legs), and your shoulders control the horses shoulders.
So for shoulder in your hips point straight ahead, your upper body (at waist) has riders shoulders IN off the rail and the horse is bent around your inside leg. The riders outside leg prevents the horse (i.e. blocks but doesn't push) the horse from evading the SI by pushing it's haunches out. It helps me to think that I'm point the way with my inside hip.
When you speak of haunches coming in (i.e. off the rail if you're riding inside an arena) that's know as a haunches in. For rider to get this come off short side and when shoulder hit long side (and haunches are still bending around inside leg and are "inside" off the rail) ask the horse to retain this position by using your outside leg behind the girth and rider sort of "cocking" hips to the inside while you shoulders point straight ahead. When the shoulders come in it's shoulder-in (SI).