Traditional Principles of Animation/Secondary action
Secondary Action is any action that results directly from another action. Secondary actions heighten interest and add realistic complexity to an animation. They should always remain subordinate (not as important) to the primary action. They should not conflict, become more interesting or in any way dominate your primary point of interest. If the audience is paying too much attention to the secondary actions, they're missing the most important element (primary action) of your shot.
It is sometimes easy to forget about secondary actions because we take such 'side effects' for granted in real life. Think of the basketball shot that hits the backboard rim. As an animator, you did a great job with the ball's shape as it hits the rim. But what about the effect the ball had on the rim itself? Did the rim wobble back and forth a little after the ball hit it? The rim wobbling is the secondary action; the result from another action (the ball hitting it).