The area behind the saddle cannot bend sideways. The spine here has stiff processes to the sides which prevent the horse from bending sideways. What the horse must do in this area, is to compensate this stiffness by lowering his inside hip and bend his haunches (lumbo-sacral joint/hip/knee) vertically so that the inner hip is actually more forward. This is part of the collecting effect of shoulder-in, and utterly necessary to keep the engagement. Without this it will only be an exercise like leg-yield, which is at its best a limbering exercise.
I mirror the spine of the horse with my hips, so I am turned a bit away from the wall. I don't ever rotate hips, waist and/or shoulders independently as that would break apart the inherent drive that I get from using my back and core. Rather, my leg will come back from the hip in order to ask the horse to move away depending on what movement I'm doing. In SI, my inside leg is at the girth and is the activity leg, outside leg slightly back to guard against the haunches drifting. In HI, my inside leg is at the girth as well, outside leg a bit back - but that is now my activity leg. In SI to R, my body position stays the same, I just move my inside leg slightly back as I make the switch and bring my outside leg slightly up - inside leg remains the activity leg throughout. HP is outside leg slightly back (activity leg), inside leg at girth - but in all of these, my hips remain perpendicular to the spine line of the horse.