The principle is dictated to by the laws of physics and follows them as much as it can; follow through demonstrates this the most with it being the law that as a character makes a movement, any loose items such as hair or an item of loose clothing will continue moving once that character has stopped due to the 'drag' on the item.
Overlapping action is simply a second action that runs at a different time alongside another action or movement. A good example of this is a walk cycle, where the arms are moving at a different time to the body and legs and therefore 'overlap' the action.
Examples:
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| Fig. 1 |
The simple follow through action of the hand continuing after the throw as it slows down displays the laws of physics and keeps the piece from becoming static, therefore making the whole action more realistic and enjoyable.
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| Fig. 2 |
Notice how the simple addition of the skirt to a bouncing ball gives it more meaning and sense of being. It adds more reference to the laws of physics by introducing the air resistance or 'drag' on the skirt and makes it more believable.
This short walk cycle demonstrates overlapping and secondary action in the characters movement.
Look how a simple standard walk is made more realistic with the simple addition of this principle; the overlapping action of the arms and follow through of the floppy ears give a real sense of weight to the character and show off his features.
Links:
Fig. 1 and 2: http://www.animationbrain.com/follow-through-overlapping-2d-animation-principle.html
Video: http://www.youtube.com/watch?v=ajagVS-M7b0
