Principles of Animation
5.Follow ThroughaAnd Overlapping Action (Cont...)
Overlapping action is when the character changes direction while his clothes or hair continues forward. 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. 
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6. Slow-Out and Slow-In
As action starts, we have more drawings near the starting pose, one or two in the middle, and more drawings near the next pose. Fewer drawings make the action faster and more drawings make the action slower.
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Slow-ins and slow-outs soften the action, making it more life-like. For a gag action, we may omit some slow-out or slow-ins for shock appeal or the surprise element. This will give more snap to the scene.
7. Arcs
All actions, with few exceptions (such as the animation of a mechanical device) follow an arc or slightly circular path. This is especially true of the human figure and the action of animals. Arcs give animation a more natural action and better flow. All arm movement, head turns and even eye movements are executed on arcs.
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