Activate the lower trapezius to pull scapula down, away from ears. Straighten the elbows by engaging triceps and feel how the long head of triceps (from shoulder blade) synergizes the action of lower trapezius.
Engage pectoralis minor to lift low ribs and expand the chest like a balloon. Draw scapulae back, then attempt to roll shoulders forward moving antagonistically. The origin rather than the insertion is what moves.