Overshoots
Much of the time, you will create key poses, and then use inbetweens to blend from one pose to the next. Blending directly from one pose to the next followling the line of action. However, sometimes you may want to Overshoot your keyframes to help create better impact or anticpation to your movements. This is particularly helpful when trying to convey extra life and movement to a scene during the character expression moments that require more drastic movement.
One easy way to think about Overshoots, is it is like Anticipation. But instead of building anticipation BEFORE a movement. You are overshooting the next Keyframe towards the END of a movement.