An action almost never comes to a complete and sudden stop. Inertia carries the object beyond the termination point, often causing the object to slowly reverse direction and settle back to the intended stop location. A golfer's swing provides an ideal example of a follow-through. Once the ball is struck, the club (and golfer) follow through the point of contact to complete the swing. What about a pencil that drops to the ground? When it hits, does it stop dead in its tracks? Or, does it bounce around a little, then rolls to a stop? Two cars collide in the middle of an intersection. Do they hit and stick? No, recoil occurs, causing them to reverse direction. Again, an action almost never comes to a complete and sudden stop.
Overlapping means to start a second action before the first action has completely finished. In life, everything moves at different speeds and at different times. OVERLAPPING ACTION is a tool used by animators to emphasize the action and mood of the character. When a character moves across the screen some parts of the body move before or at different rates than others. Some parts of the body will LEAD the action and some parts will FOLLOW the main action.
Tip: Nothing should come to a complete stop before starting the action of another part.
Great Resources talking about Follow Through and Overlapping Action:
Breaking of Joints
To create flexibility and more interesting motion when a character moves its limbs, sometimes animators "break" the joints. This means they bend the elbow, knees and other joints in ways that are not physically possible in the real world.