Add an iscube and and place it behind the person and parent link it to the person root control. Then add another smaller cubea short distance behind that and parent it to the shoulder control. Set a collision trigger between the two cubes to trigger the pose change. When you pull the shoulder it will move the small cube forward to collide with the larger one, just play with the distances to get the interaction set how you want it. You can then use CheesyFX Collision manager plugin to prevent the cubes interacting with anything else and set their alpha to -1 to make them invisible. You could set up multiple versions of this positioned to the right angle for each pose and have the trigger switch their collisions on or off depending on the pose.