Slow in and Slow out

These are necessary in order to create a spirited feel as it involves planning out the timings of in-between extreme key frames in order for them to appear to flow naturally.

Slow In
This describes an action decelerating into a key-frame. This normally occurs at the end of a sequence when the opposing force begins to effectively act upon an object and it is slowing down to a rest, a lighter object will do this more quickly than a heavier one.

Slow Out
This describes an action accelerating out of a key-frame. This normally occurs at the beginning of an animation as this is usually when enough force is applied to overcome the object or figures inertia. The heavier the object the longer it will take to build up the momentum.
The in-between frames needed in these techniques are named cushions. The more cushioning there is the longer the acceleration and deceleration periods. Sometimes these processes can be referred to as ease in and ease out, and can be described in some texts as being the opposite way round.