Follow-Through and Overlap actions are to some degree part of the same phenomenon.
Follow through and overlapping action is a general heading for two closely related techniques which help to render movement more realistically, and help to give the impression that characters follow the laws of physics.
Following through means that separate parts of a body will continue moving after the character has stopped. Overlapping action is the tendency for parts of the body to move at different rates (an arm will move on different timing of the head and so on).
A third related technique is “drag”, where a character starts to move and parts of him take a few frames to catch up. These parts can be inanimate objects like clothing or the antenna on a car, or parts of the body, such as arms or hair.
On the human body, the torso is the core, with arms, legs, head and hair appendices that normally follow the torso’s movement. Body parts with much tissue, such as large stomachs and breasts, or the loose skin on a dog, are more prone to independent movement than bonier body parts. Again, exaggerated use of the technique can produce a comical effect, while more realistic animation must time the actions exactly, to produce a convincing result.
Follow-through is the movement following the termination of an action. When kicking a ball, for example, the foot’s swing doesn’t stop the second it contacts the ball: it continues in an arc. Similarly, when a car stops abruptly, the passengers in that car are still thrown forward – they do not stop at the same time as the car.
Overlapping actions are those that begin before previous actions have finished, or where the direction of movement changes – some parts may continue on their original course until they catch up with the change in direction.
As a general rule, the targets for these principles divide into two main groups: active and passive.
The passive group is anything that is driven purely by adhesion, constraint or influence from an initiator – for example, hair attached to the scalp by its roots or clothes wrapped around the body.
They have no choice but to follow the movements of their initiating force, and react mainly to gravity and atmosphere. Those in the active group have some control over their movements. They include arms, legs and tails: objects that can be passive, but also have the ability to behave in more controlled ways.
The exaggeration of these principles is dependent on their weight and their environment. Under water, for example, passive objects waft about in a different manner to on the surface. A common mistake is making things look swimmy – like they’re underwater when they’re not.
From Frank Thomas and Ollie Johnson:
Drag in animation, for example, would be when Goofy starts to run, but his head, ears, upper body, and clothes do not keep up with his legs. In features, this type of action is done more subtly. Example: When Snow White starts to dance, her dress does not begin to move with her immediately but catches up a few frames later. Long hair and animal tail will also be handled in the same manner. Timing becomes critical to the effectiveness of drag and the overlapping action.