... Anticipation

The use of anticipation prepares the audience for the next action about to occur and is generally only a couple of frames in length. Combined with exaggeration, in animation form, the motion is often more obvious however in real life movement there is anticipation e.g as a person goes to walk forward their body leans forward, as when a dancer is to jump off the floor, their knees bend first in preparation for the move.

In the example included of a baseball being thrown, the ball does not just leave the throwers mitt, his whole body moves in anticipation for the action, moving backwards then lunging forwards while his legs and hips move also before the ball has even left his hand.

Slow in Slow Out

Slow and Steady

The movement of the human body, and most other objects, needs time to accelerate and slow down. For this reason, an animation looks more realistic if it has more frames near the beginning and end of a movement, and fewer in the middle.This principle goes for characters moving between two extreme poses, such as sitting down and standing up, but also for inanimate and moving objects.

Animation without Slow In and
Slow Out between the key frames appears abrupt, mechanical, and stiff.

Slow In and Slow Out also
relates to the weight or mass of an object. An oil tanker takes longer to get up
to full speed and longer to stop than a speedboat will.

Let us see how the Big Boy camp did..

the Big Man has gas!

Follow Through and Overlapping Action//.

In animation, certain parts of an animation must keep moving even after the main action has completed, for example:

when a woman swings her head, her hair continues moving after she stops moving her head, and this hair motion may continue even as she begins her next action.

Apparently not only women flick their hair... rats do it to: