Here’s a great demonstration of a drill that NFR Qualifier Melanie Southard uses to achieve the shoulder and rib cage control she looks for in her horses.
She sets up 3 or 4 barrels with a cone in the middle. You will see her to do a regular turn around the barrels, but do a counter arc when she goes around the cone that is in the middle. It’s important to listen to all the points she makes at the beginning as it sets you up for proper positioning and your own hand control for this exercise.
This is a lot of work for a horse mentally and physically. You may opt to introduce this exercise with just a few turns the first time and build up to asking for more like Melanie has done here.