Re: Principles of Aiki
I have had similar instruction with regards to the shoulder blades from a Russian Kettle Bell instructor. His visualization was to try to drop the "points" of the shoulder blades into the opposite back pockets (left shoulder into right back pocket, etc.).
While he has not trained in Aikido that I know of, he is proficient in many other styles of martial arts. His assertion was that this action activated the lattisimus dorsi. This pulled the shoulders into a more secure position in the shoulder "socket" and forced the lats to acts as additional shoulder stabilizers. This would create a more secure connection of the shoulders to the axial spine, meaning that axial motion is translated more powerfully through the arms. In this sense, the arms are merely conduits and connectors to the axial body's power. He used the example of strikes, but I can see how this would also translate in much more efficiency in Aikido techniques as well.