Seeing as the head is at the top of the body (usually) we started with that.
All poses may not be as active so here is 'leading with the head' a little more naturally.
Just like the song, "Head, Shoulders, Knees and Toes", shoulders are next. Shoulders tend to be pretty aggressive if you think of breaking a tackle in football or body checking in hockey or a space warrior babe alien thing.
But shoulders also act as shields to try to protect feelings or the spirit inside as in the case of a shy person as suggested.
Working our way down, we get to the stomach. I going to digress for just a moment and say, I had a room mate, a well fed room mate, back in college who lead with his tummy. If you were having a conversation (or doing any sort of activity) The tummy would arrive first, leading the 'mouth' which would drop whatever opinion the room mate had at the time. Below are just two examples of stomach lead.
Leading with the knees tends to push the body backward awkwardly, this is great for describing a lanky or under-exercised figure. But, if you look at people taking a photo on their cell phones, they tend to lead with the knees to try to angle the body back to get more into the shot. If you look at "the Lovers" from Commedia del'arte, you will find that these characters (who usually lead with the chest) may fall back to lead with their knees when they stand still and let the warm emotions of love and happiness wash over them. Same is true of people who are experiencing a true emotion.
Leading with the toes lightens the character's movements. A sneak, sashay, or sexy walk (though a sexy walk also is a lead with the chest and shoulders as well) are examples of leading with the toes.