Arcs

All actions that are performed in animation are followed by an arc or a circular path. These arcs are clearly visible in the graph editor and are separated by key frames. Arcs in animation give a more natural action and better flow. Few examples of arcs are head turns, eye movements, arms swinging, balls bouncing, pretty much anything that is moving.

Reference Links:

http://www.evl.uic.edu/ralph/508S99/arcs.html

http://minyos.its.rmit.edu.au/aim/a_notes/anim_principles.html