This Animation Tutorial covers the process of drawing extreme frames. They serve as a point of reference as to where major changes or positions are taking place in an Animated scene and ultimately serve as an unbeatable foundation for a solid piece of character animation.
Once these extreme poses are established the animator can then go about breaking down the action between these positions by drawing primary poses that solve any technical problems or difficult motions from getting from one pose to the other. These will be the key frames. And they will indeed be the framework of the scene but a framework within a foundation of extreme positions.
The breakdown drawings will be made and the animation will be timed and charted and finally inbetweened.