Lateral positions are often quite challenging for students to grasp. The marker points to view are the horse’s head, their shoulders, and their hips. The ground line is flat, and thus the differences in width of those three markers is simpler to see. Shoulders are narrower than hips, and the barrel flairs wider from just behind the shoulders before narrowing again just before the hindquarters.
In the most basic discussion of shoulder-fore, you are attempting to line up the horse’s inside shoulder with their inside hip. In our lying down horse visual, that would be the same as the ground side of the horse being the shoulder-fore position. You will note that the bulge of their rib cage is to the top (outside). Therefore, to ride shoulder-fore, the rider must encourage the horse to swing their rib cage outward and around the rider’s inside leg. This creates the “bend” that is so valued in encouraging horses to engage their thrust from their inside hind leg towards the rider’s outside hand.