Animation - Principles 7 - 9: Arcs, Secondary Action and Timing

Arcs - Not all motion is linear and most natural motion follow a curved, circular path called an arc.
The motion is usually constant on one axis whilst another axis is slowed in and out midway to get that bounce effect. actions that follow straight lines tend to look more dull and lifeless as opposed to having the appendage follow an arc.

Secondary Actions - The secondary animation is an addition to a primary motion, giving it more dimensions and gives the audience more information about what's going on in the scene.
Secondary actions can take place before, during or after a primary motion but it's important to consider staging and what should be the key focus at a given time where to not overwealm the viewer. It's important to ensure the secondary doesn't obscuring the primary motion and blocking out any key emotions.

Timing - The emotion and purpose behind a motion is greatly influenced by the number of frames between each key motion. More frames even spaced translates to a slow motion whereas less frames creates a faster motion.
Say the frames per second (fps for short) for your animated production is 24 or 30 and you're animating every frame, not only is it more time consuming to animate every frame but it can often look too smooth. This is what's known as animating on 1s.
Animating on 2s is drawing every other frame which is more efficient and can even result in a more lively motion. only needing to animate 12 or 15 frames per second is a signifcant reduction in the long run and can cut back how long it takes to finish an animation.