Think about the Neck bone. In our bowing action, what causes the neck-bone to move? Well, it moves because of the displacement of the torso: the torso moves forward, which causes a force to start pulling on the neck bone.
The key thing to remember is this: the neck bone does not begin to accelerate until the torso has already moved. So, the neck bone's movement should lag somewhat behind the movement of the torso. Likewise, in theory, the head should lag somewhat behind the motion of the neck.
While the physical principle is complex, the application to animating is actually quite easy. Basically, the principle of Overlapping is this: everything does not start to move and stop moving all at once. One thing starts to move, and then another, and then another.
This serves its purpose of enhancing realism, but can also serve a more subtle purpose of leading the audience to anticipate future actions. If you have a constant series of overlapping actions, then there is never a time when all of the actions are stopping. It's always a matter of one action stopping while the next starts.
So, by the time the audience is finished watching action number one, they have already started to perceive action number two. There is no gap in perception where they don't see what is coming next.
My guess is that, by now, you're eager to apply overlap to your bow. Let's go on to the next page and learn how.