Slow In and Slow Out is a principle of animation that brings realism and believability to movement. It describes how most motion in real life doesn’t begin or end suddenly.Instead, characters and objects start to speed up at the beginning of an action, and decelerate toward its conclusion. For instance, when a person tosses a ball or operates a vehicle, they don’t immediately attain maximum speed or abruptly stop; there’s a gradual acceleration and deceleration. In animation, this principle is made by positioning additional frames at the beginning and end of a motion, while reducing the number of frames in the center, where the speed is greatest.

Utilizing Slow In and Slow Out gives animation a natural quality, and it makes it feel less robotic. It improves the weight, timing, and rhythm of a character’s movements and directs the viewer’s focus to important poses. Without this principle, a character’s motion could seem unrealistic. For example, an arm might “snap” into position rather than smoothly moving there.