As action starts, you may have more drawings near the starting pose (1), 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.
The first timing chart shows a slo-in using half and a quarter inbetween from drawing #1. The total elapsed time (if shot on twos) is 8 frames or 1/3 of a second (at 24 frames per second).
The second timing chart shows a slo-out using a half and quarter inbetween moving into drawing #4.
The final timing chart shows both a slo-in and a slo-out. Drawing #5 is the halfway inbetween. In this timing chart, there are more drawings (3 to be exact) moving into the slo-in and out. This will slow the action down considerably. The total elapsed time (if shot on twos) is 18 frames or just over 2/3 of a second (at 24 frames per second).