Alan Becker breaks down key elements of animation from Thomas and Johnston’s Disney Animation: The Illusion of Life. Recommended.
Slow in, Slow Out; or, (Sine) Wave as you Pass
Rule Six (“Slow in, Slow Out”) is a nice articulation of sine-wave movement:
Sine is a natural sway, the epitome of smoothness: it makes circles “circular” in the same way lines make squares “square”.Better Explained
See also: Better Explained’s sine-wave simulator and comparison of sine-wave movement vs linear movement – if you haven’t noticed before, marvel at how sine-wave movement in two axes draws a circle.