Slow In + Slow Out (easing)
As action starts, we have more drawings near the starting pose, one or two in the middle, and more drawings near the next pose. Fewer drawings make the action faster and more drawings make the action slower. Slow-ins and slow-outs soften the action, making it more life-like. For a gag action, we may omit some slow-out or slow-ins for shock appeal or the surprise element. This will give more snap to the scene.
Inbetweens which are closely spaced will move slower than those spaced further apart. If you space most of these drawings close to the start of an action and progressively space them further and further apart towards the end, the action will start slowly and build to a punch. The opposite will be true if most of the inbetweens are spaced close to the end – the action will come to a gentle halt. This variation in spacing is called ‘fairing’ the movement, or ‘slow in’ and ‘slow out’ or ‘ease in’ and ‘ease out’