Squash and Stretch
When an object squashes and stretches, it appears to be composed of a softer, organic, elastic material. Objects which do not stretch appear rigid and plain.

Click on the balls to the right, to play them. Although the Ball #1 bounce is more realistic, it's looks unappealing.

By stretching Ball #2 before and after impact, and then squashing it as it hits, it appears to be moving faster right before and after it hits the ground. The stretching and squashing evokes superior optical stimulation.

Subtle exaggeration can grossly enhance an animation's illusion of realism, and make it more stimulating to watch. Overexagguration can make an animation look cleary unrealistic, and cartoon-like.

For most natural animation, good to keep exagguration quick and subtle.

Direction and Flow...