'Animation is all in the timing and in the spacing' [Grim Natwick]. Timing is - PacMan goes to town/how fast does he get there (and in detail - in how many moves)? Timing is all about computing, measuring your beats, your seconds, etc. Simple. Spacing is more complicated - what path does PacMan follow to get in town (and in detail - what kind of moves)? Great spacing, I think, makes the difference between OK-animation and WOW-animation. Quoting Richard Williams - "good animation spacing is a rare commodity".
So animation is all: timed paths of action. Every animated joint has its own path, and there's an entire hierarchy of joints, some more important, some less, some of them leading (like the wrists of a character, or the hips, or the ankles), most of them driven by those leaders. If there would be no hierarchy it would all be a mess, but, luckily, we've got all nice and clean and organized.
But I won't talk about that. My question is: how do you know what the appropriate path of action is??? How do you actually calculate that timing and spacing? In a coherent way, of course :) and part of this is being aware of your weights, at all times. The body is balanced around a center of weight, and in motion this center becomes dynamic: it moves too!!
I'll talk some other time about what I like to call static vs dynamic balance (balance in movement is a different beast from static balance!).
When I say that the weight moves I don't mean - it moves along with the body. Nope, it doesn't. It's got its own rules, it's own movement. Weight moves differently from the overall body movement, but not independently. The center of weight can be a driver and can be as well driven. Usually it starts as being driven and, as it gains momentum, and it's movement overruns that of its driver (the body), it becomes the driver. When we start moving we put the weight in movement, and the more weight the character has, the more difference weight-movement/body-movement we have. Meaning that a heavy character will start its weight slower, but once it's moving, the weight has a lot of momentum/inertia and its hard to stop. If he tries to stop, the weight will become a driver and drive the character in the direction it was going. And it will take a long time to stop. In this case the difference what-the-body-wanted/what-the-weight-wants is great. But in a light character this difference is small, to the point where it might even become unnoticeable (on an ant I guess).
We can imagine this weight center as a ball, and animating the weight as ball-animation. We can include squash and stretch if we want, and so the weight's squashing and stretching defines the character's squash and stretch, or just the overall body pose (squashing and stretching doesn't apply to deformations only , but to gestures as well, which is what John Lasseter has been preaching since the very beginnings of computer animation).
I'll illustrate the weight center moving inside the body with my simple character - Josh. Josh is standing in example 1, he doesn't do anything. His weight is at default position, in perfect balance.
In the 2nd example an external force pulls Josh down, but his weight is still up, it didn't start moving yet. This means the relationship body-movement/weight-movement is changed. (This could also be an example of Josh up in the air, trying to go down, but his weight is still up, it didn't started following yet.)
In my 3rd example, Josh lets himself fall, which means that the weight leads this time. The body follows, and the hands (auxiliaries) are last. (This could also be an example of Josh trying to go up, but his weight is still down, it didn't started following yet).
These are exaggerated examples of how the weight center shifts in relation to the character's body. You can pull Josh in any direction - his weight would follow in a short while... Of course, he could be running and his weight would pull him back. Imagine you could take this to an extreme, like it happens in many cartoons, and have him running faster and faster, without being able to start. Like a slingshot.
I dind't squash&stretch his weight-ball, but you can do that :) Actually, animating the weight-ball, with or without squash and stretch, is an embryo of animating the character itself.