Okay, so here's the concept I actually wanted to talk about concerning "squash and stretch": how it applies in drawing (as opposed to as a force in animation).
Basically this is all about using "Squash and Stretch" to help organize your drawings and as an aid in drawing forms that are fleshy or pliable.
When you bend over and touch your toes, you feel a stretching sensation across the back of your legs, your back and spine and the back of your neck, right?
And you feel a squashing sensation in your stomach, right?
This can be helpful in drawing. When one side of a form stretches, the other side side should squash...and vice versa. This really helps add life to your drawings, get a better feeling of weight and is an aid in avoiding symmetry. A drawing that is totally symmetrical (both sides are mirror images) is usually a flat, uninteresting drawing. Here's a refresher from Carson Van Osten on why symmetry leads to weak drawing:
I find myself thinking about Squash and Stretch while I draw and I often organize my figures so that one side is always squashing and the other side is stretching. And the reason that Rapunzel makes a good model to illustrate this is that I used that concept a lot while drawing her. It helped me organize all that hair in a way that prevented it from just being a shapeless mess (well, sort of).Click to see bigger:
Here, I've drawn red arrows to indicate the stretches...and invariably you'll find a blue arrow on the opposite side of the body part indicating a squash.