The 5th principle of animation. Overlap will give life and fluidity to your characters by breaking up the stiffness and making your motions more interesting.
Animator's Survival Kit - Richard Williams
Overlap is related to drag. Parts that are "further down the chain" will drag (lag behind) more than the ones closer to the main joint. This is what we call successive breaking of joints. In the image above, the main joint would be the shoulder, since that's where the rotation of the arm originates from. The wrist is at the bottom of the chain, so it lags furthest behind the main motion.
Understanding Overlap
"'Things don't come to a stop all at once, guys; first there's one part and then another.'" -- Walt Disney, The Illusion of Life by Frank Thomas and Ollie Johnston
Overlap is primarily added in your breakdowns. (For more information on breakdowns see this post: https://rbossert27.wixsite.com/mysite-2/blog/breakdowns-how-to-get-from-here-to-there.) The best way to explain this is using a pendulum. Once you have your two main keys (A and B in the image below), you can place a breakdown in the middle, where the joints further down the chain are still rotated back, favoring their previous position, whereas the joints closer to the top of the pendulum are rotated forward, favoring their next position. This will often create a S shaped breakdown, which is much more interesting than the straight inbetween that you would get if you let maya compute the exact middle position between pose A and B.
Another good example of this is in a blade of grass. The tip of the grass blade will lag behind more, favoring it's first position much longer and reaching it's next extreme pose after the base of the grass blade has already started to move back the other way.
Keith Lango - Overlapping Action Tutorials
Below I linked to the first of a set of three videos from Keith Lango covering the subject of overlap, which completely changed the way I thought about this concept. The most insightful part of these videos was how overlap can be applied to the human body. Arms and legs of course are the most obvious use of overlap, but it can even be applied to a character's spine, which Keith covers in the third video.
Overlap in Hair and Cloth
"To make even the dullest action or figure interesting, we break the body into sections - into different entities and move sections - one at a time, constantly overlapping." -- The Animator's Survival Kit, Richard Williams
The video below, by Aaron Blaise, gives some great examples of how overlap can be used in human and even cloth animation. The biggest takeaway from this video is that while an object can overlap within itself, it can also overlap behind the main action. This is common in cloth and hair, since it has a delayed reaction to the body's movement. Typically - depending on how heavy the cloth, hair, etc is - as the body goes up the cloth/hair will go down and vise versa. Aaron also talks about how overlap and follow through are related and work together to create realistic movement. (For more information on follow through see THIS POST.)
Summary
Overlap can be understood through the phrase "successive breaking of joints", and it is often added in your breakdowns. It can be found in the limbs and spine of a character as well as cloth and hair. If you understand this principle, you will be able to add a lot of life and believably to your characters. If you want some practice with overlap, I recommend trying out a pendulum assignment or even animating a blade of grass. Start with something simple and work your way up from there, especially if you're just beginning to learn the 12 principles. These principles are extremely important as you progress in animation, and I have found that starting simple allows you to focus on and learn one subject really well. As tempting as it is to take on more, I promise you that you'll get a lot out of a simple assignment like a pendulum. Good luck, and happy animating!
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