Answer:
A beautiful metaphor to explain the necessity of a solid but vivid structure and a logic order while writing a speech.
Explanation:
The idea of thihes metaphor, like all metaphors, is to visualize the topic - in this case writing a speech - in order to have a clearer idea on how to proceed.
Following this methaphor we could start with the head and introduce the topic in a few sentences of well selected words. When you move downwards to the middle and extremities you might go into some more details, using the arms and legs as a means of subdividing your topic.
Your feet, obviously, are important to keep you on the ground and not get lost in airy idealistic observations.
And remember, the human body is a perfect machine and very lively.