In classic animation terms this would be called “successive breaking of joints.” However I find the label unhelpful. The principle is basically a type of overlapping action applied to a character’s moving parts. What does that mean?
If Joe is standing still and decides to turn and walk off, the first joint that “breaks” would probably be his neck to turn his head (in fact his eyes would lead his head.) Now his head movement will lead the next joint which would be his torso, followed by his hips, which is followed by his leading leg lifting to counter balance his weight. The joints don’t all start to move at the same time. It might be 8 frames after the head move that the hip makes any significant rotation. That is successive joint movement.
Add into the mix his arm movement, which could follow the neck. Perhaps the shoulders lead the torso. The elbow joint follows, then the wrist, then the fingers.
Successive Joint Movement Convey’s Character
This principle is not merely a operation of animation mechanics. It also conveys emotion and personality. If Goofy leads with his feet this could suggest he’s anticipating an opportunity to sneak away. If he struts around with his torso leading his arms so strongly that the arms appear to dangle, this could convey laziness or some sort of cocky, carefree attitude.
Leading with the head gives the impression of alertness. Leading with the shoulders or hips are most common and natural. Leading with the feet and hands is more common for sneaky situations or if the character is being led by another character, such as taking one by the hand.
Equivalent Joint Movement
If your character doesn’t have successive joint movement they will appear wooden and stiff. This is equivalent joint movement, which is unnatural. It’s a special effect. Characters almost never start or end any motion with all of their joints starting or ending at the same time. We see this when the Road Runner comes to a stop. We see this as an exaggerated effect when a character snaps to attention in the presence of a General, or if they have just been caught with their hand in the cookie jar.
Breakdowns
One of the key purposes of breakdown poses is to define successive joint movement and overlapping action between key storytelling poses. How a character reaches these poses and then leaves them is the purpose of the breakdown, and they must account for how the joints succeed one another. And these are just as important for conveying personality and emotion as any key.
