every motion has a path of action - i.e. an arc
most things in nature move in arcs
arcs are used to make a movement much smoother and less sharp/unnatural e.g. the trajectory of a bouncing ball
all head turns, limb movements and even eye movements are executed on arcs
when the head moves, it tends to lift or drop slightly
eyes move very fast and thus have fewer frames in between
you can create unnatural-looking eyes by having the blink happen one eye at a time
joint 'breaking' makes actions more flexible and limber/fluid
always remember what moves first (which joint?)
this not only creates more believable arcs, but also smoother overlapping actions
in arm movements, the shoulder leads first, then the elbow, then wrist and then the hand is last
when something has a ripple-like movement (e.g. a whip, grass in the wind, hair, cloth, tail, seaweed etc.) it has C and S curves
this is also sometimes referred to as the Wave Principle