Arcs
Arcs help to create a fluidity of movement in animation. Understanding the intended path of an object will make the final motion more believable. An arc is a curved line drawn to assist the animator, and can be implemented on several aspects of an object, if several parts are moving.
On any human action, there are many parts to consider. Even if there is a main action you're thinking about, i.e. a football kick, the head will be making its own path, the arms, the hips, the shoulders...
All these aspects are communicating something, and although it's a lot to think about, it can make the process more interesting to pay attention to this and add more character.
Here's a good example of an arc (right), showing a strong upward punch. The consistent curve will help create a smooth motion. The next part to consider is any additional arcs to give the character more weight and convincing form. For instance, what is the other arm doing during that action? He is turning his back to us, so what arc are his shoulder taking to get to that point?
There are always little details that the viewer will notice if you put the work into it. This kicking animation shows how subtle some actions can be but we do recognise the balance and weight that it communicates. If there is an extreme action in one part of the body, it's likely that other parts will counter-react, even in the slightest way. Using arcs here make that downward bounce fluid and readable.