12 Principles of Animation: Slow-Out, Slow-In, Ease-Out, Ease-In

Easing-in and easing-out presented with a coordinate graphical plain

Slow-Out, Slow-In, Ease-Out, Ease-In, whichever works best, is an aspect of animation that deals with the smoothness of starting and ending actions, how well they flow and progress. In other words, the slow-out and slow-in technique is used when manipulating a character or form that prepares for movement, or prepares to terminate said movement. Without this technique, character's movements might appear a little mechanical, distancing them from the achievement of life-like motion (as robots). The way this technique is done is quite planned out: the first, middle, and last frame of the movement are created, and then during the incipient or final phase several images are drawn in-between to suppress the beginning and closing action of movement.

Tom and Jerry: https://www.youtube.com/watch?v=c06HUhCAn9k

For example, after Tom slaps on the collar to Spike's waist, he proceeds to run away with Spike breaking away from the snare and then chasing him. When Tom starts running, he doesn't mechanically achieve the same speed he would run away with, but first gathers it, as there are several more frames in between him getting ready to run away... same goes with Spike.