When a body moves, its parts do not move together. The hips lead, the chest follows, the head follows that, the hair follows last, and each part continues briefly after the one before it has stopped. Those delays are what make motion read as belonging to a physical object rather than to a rig, and they are the most reliable improvement available to any animated figure.

The principle is old and well documented, and it is applied badly more often than it is applied wrongly. The usual errors are too much, uniformly applied, or delay without any settle at the end.

Drag, overlap and settle

Drag is a part starting later than the part driving it. Overlap is parts arriving at different times rather than together. Settle is the small continuation and return after the main motion has stopped. All three are aspects of the same delay and it is useful to name them separately, because a shot can have one and lack the others.

The most commonly missing one is settle, which is what makes a stop read as arriving somewhere rather than as the animation simply ending. Two frames of overshoot and three of return will do it, and it is the cheapest fix in the list.

How much delay

Two to five frames between a driving part and a following one covers most cases at ordinary playback rates. Heavier or looser parts take longer; tightly attached ones take less. Applying the same delay to everything is what produces the wobbling, gelatinous look that people describe as too much secondary motion.

Varying the delay between parts is what makes the body read as a system of connected things with different properties. It is also what defeats the eye's pattern detection, which matters particularly in looping work, as described in the piece on making loops hold up.

Amplitude follows mass and force

How far a part travels should depend on how heavy it is and how hard the body moved. A small motion produces almost no secondary movement, and animating a large one anyway is the second commonest error, since it implies a force that the main action did not supply.

This is the weight argument from the piece on floaty motion. Secondary motion is evidence about mass, and evidence that contradicts the rest of the shot makes the whole thing less convincing rather than more.

Where it becomes noise

Every part moving all the time produces visual noise, and noise competes with whatever the shot is actually about. Choosing two or three elements to carry the secondary motion, and keeping the rest quiet, reads as considered and directs attention where it belongs.

Hair and loose clothing are usually the right choices, since they are large, visible in silhouette, and expected to move. Adding it to everything simultaneously is the animated equivalent of uniform detail described in the piece on repeating patterns.

Simulation versus hand animation

Solvers produce secondary motion automatically and produce plausible rather than expressive results, which is the trade described in the piece on cloth and hair. For a hero shot, hand-animated secondary motion aimed at a specific reading will usually beat a simulation.

The practical compromise is to simulate, cache, and then adjust the few moments that matter. That keeps the tedious accuracy and restores the emphasis, which is the same division of labour recommended for cloth generally.

Secondary motion in drawn animation

In hand-drawn work the same delays apply and they are drawn rather than solved, which means each one is a deliberate decision. That is an advantage, since a drawn overlap can be simplified into a few clear shapes rather than a mass of accurate detail, which is what stylised work wants anyway.

Working on twos or threes makes the delays coarser and the effect stronger, since a two-frame offset at twelve drawings a second reads more clearly than the same offset at twenty-four.

Making delay do the work

Offset following parts by two to five frames, vary the delay between different parts, scale amplitude to mass and force, add a settle at every stop, and restrict secondary motion to two or three elements. That is the whole technique, and it improves almost any shot.

Characters here are fictional and unmistakably adult, as set out in the reader agreement.