Once an animator has finished his 'extreme' character poses and he assured to him self that the frames are the best he can possibly do for the audience to see.He has to time all the key frames to move quickly from one to another, so the rest of the footage of the scene would be as close as possible to the key frames
By drawing in-between frames close to each extreme key pose frame, the animator will achieve a flowing animation motion. Using closer in-between frames to the extreme frames or further frames from the key frames, the animator uses a technique called Slow In and Slow Out.
Too much of this would give a mechanical feel to the action, taking away the action and life to the characters and the entire scene. Together with staging it was a very important discovery.
Walt Disney kept asking the animators to analyse actions more and more and to understand how the bodies are working.
Animators back than could not believe that anyone would be so interesting in the mechanics of motion, but this approach was the very core of the technique.