Never too late to answer, Chris, you are very close to the answer,

you are correct, it isnt the shoulders, or teh legs, they move after you use other parts of your body,

The Japanese explain it as your Hara.

your stomach muscles are the first part of yor body wich tighten or loosen when you move forward, and yes if you could see taht happening it would be great, you can tell where you oppoent is moving form the grip. your oppoenent shoulders will give away a lot of thier intentions, and you will be able to determine a suitable counter or strategy to defend,