Isolate and Articulate
This standing warm-up isolates the rib cage and spine and finds the contrast between engaging and releasing. From a wide plié you link your hands and let the ribs fall open, without forcing them. Then your abdominals pull you in for a whole eight count, until you are shaking.
Later the ribs travel one at a time: down the side, around the back, in front. It may feel strange at first. Go very slow, and make sure no other body part is helping.