Agree on your sources of 'power'*, the bigger, stronger and slower muscles are the sources of power. Most of that power goes elsewhere but a portion can be transmitted into the arm and racket.
The shoulder transmits more of that power to accelerate the arm and racket effectively if it is tight against the chest. But if it is not tight the transmission is not as good. That is how it looks to me.
I have experimented recently and have begun to study high speed videos of pro players and believe that an important part of the 1hbh is to have the upper arm tight against the chest.
* "Power'. For tennis strokes, I understand and can measure velocities much easier than 'power', which I can't understand well or measure.
Last edited by Chas Tennis; 05-08-2014 at 06:59 AM.