For my animation principles presentation on the 12th, I have to focus on 'Follow Through' and 'Overlapping Action'.
Everything doesn't move at the same time. A character does not stop after finishing an action and then move to do the next. Hair, ears or loose skin, for example, will fall behind a few frames due to only being attached to the main action rather than part of it. How it moves depends on other principles such as weight, sometimes referred to as 'Drag'. This is overlapping action.
To 'Follow Through' basically refers to the movement following a main action. For example, when a character stops moving, everything doesn't stop at once. Any clothes, loose skin, hair or even limbs make take a few frames to catch up and reach their stopping point because they may move individually to the skeleton.
Here is a short, simple video on this principle:
I also looked up the principle in 'The Illusion of Life: Disney Animation' by Frank Thomas and Ollie Johnston:
I watched 'Mickey's Grand Opera', particularly Donald Duck's movement (though the principle is present in all the characters shown) and you can see that though he holds his position sometimes he doesn't actually stop moving. His arms are usually doing different things from each other and his legs are walking this way and that. By following this principle his movement is much more life-like, or more accurate to how movement works in real life.
Richard Williams' 'The Animator's Survival Kit' very simple but effectively explains Follow Through and Overlapping Action:
For my presentation, I am thinking of including this particular image to illustrate the principle. I have added my own notes over the image below to show the flow of movement. I may draw over the clean image in my presentation as I think that would be the most effective and interesting way to explain the principle.
References in order:
# Animation Training Video. Linked from Youtube, created by http://www.animatortraining.com
# Pages from 'The Illusion of Life: Disney Animation' by Frank Thomas and Ollie Johnston.
# 'Mickey Mouse: Mickey's Grand Opera' video. Linked from Youtube, created by Disney, 1936.
# Pages from 'The Animator's Survival Kit' by Richard Williams.