Is this about the ‘Lower Body Machine’ (LBM) or the ‘Upper Body Machine’ (UBM) or both? This learning moment is really about your ‘Total Body Set-Up’.
If one thinks about the Vertebral Column or Spinal Crankshaft, “Thoracic” is about The Chest and Shoulders. The Scapulae are the ‘Shoulder Blades’. If one pulls the ‘Shoulders’ far aft, rather at attention, this Musculo-skeletal action reduces the separation or space between the Scapulae. Your 555 Team refers to this condition as ‘Cracking Walnuts’. ‘Time For A Feel’? Get a golf buddy to stand in a good overall ‘Set-Up’ with shoulders neutral and ‘Lever Assemblies Hanging’. You will feel as though your ‘Shoulders’ are forward towards your ‘Sternum’.