During the 1930s, animators at Walt Disney Studios developed a list of 12 basic principles of animation through which to achieve character and personality through movement. These principles were laid out in The Illusion of Life by Frank Thomas and Ollie Johnston. #6 is “slow-out and slow-in”:
As action starts, we have more drawings near the starting pose, one or two in the middle, and more drawings near the next pose. Fewer drawings make the action faster and more drawings make the action slower. Slow-ins and slow-outs soften the action, making it more life-like. For a gag action, we may omit some slow-out or slow-ins for shock appeal or the surprise element. This will give more snap to the scene.
Animator Vincenzo Lodigiani recently visualized the principles using a simple cube shape. You can see them individually here or all together in this video:
In a nod to the increasing prevalence of animation in app design, Khoi Vinh notes:
It’s a good reminder that as the overlap between interface design and animation grows wider, designers would do well to take note of the many decades of insight and knowledge that animators have accrued.