Drivers: Wave, Spring, Link and Audio level

Add a driver

Select the object and find the property in the Inspector. Any number that animates can take a driver, including Position, Rotation, Scale and Opacity.
Open the row's More menu, or right-click it, and choose Drive with. Pick Wave, Spring, Link or Audio level. For a property with several axes, Wave asks which: X, Y or Z.
A card opens with the driver's settings at their starting values. Adjust them and press Add driver.

The row then reads "Driven by" and the driver's name. Wave and Spring work on top of the property's own keys, so you keep the motion you set and the driver adds to it. Link and Audio level set the value themselves.

A choice that can't work yet is greyed out with the reason, such as "Add an audio clip to this composition for an Audio level to listen to." A property that already has a driver or an expression says "Remove this property's driver or expression to drive it another way."

Wave

Moves the property back and forth around its own value. On one object it's a steady wave. On a chain of parts, such as a tail, a tentacle or a row of letters, each part's wave trails the one before, so the whole thing ripples.

Amplitude: How far the wave moves each part from its own value, in the property's units: degrees for Rotation, pixels for Position. It starts at 10 for rotations, lengths and percentages.
Frequency: Cycles per second. Starts at 1.
Phase per segment: How far each part's wave trails the part before it, in degrees. Starts at 0, where every part moves together.
Falloff toward the tip: How much smaller the wave is on the last part than on the first, in percent. Starts at 0.
Start time: When the wave starts.

To wave several parts, select them in the Timeline or the Viewer, then choose Apply to selected parts from the card's Wave actions menu. The wave moves them in order, its phase stepping from the first to the last. The card's Apply to list switches each part on or off.

Spring

Makes the property follow its own animation like a weight on a spring: it lags, swings past the target and settles. Key a move with two keys, add a Spring, and the move gets follow-through without more keys.

Overshoot: How far the spring swings past its target, in percent: 0 follows without overshooting, and 100, where it starts, is the full swing. It goes up to 200.
Frequency: How fast it swings, in cycles per second. Starts at 1.
Decay: How long the swing takes to die away, in seconds. Starts at 1. With no damping it never decays, and the field reads ∞.
Start time: When the spring starts.
Advanced: Mass (in kilograms, starting at 1), Stiffness and Damping: the numbers Frequency and Decay are made from. A stiffer spring swings faster; more damping settles sooner.

Link

Sets the property from another value in the composition, mapped from one range to another. Turn a dial and a needle follows; move a slider and a door opens.

Source: The property to follow, from another object or a numeric property.
Input range: Two values of the source. The first gives the output's first value, the second its second.
Output range: The values the driven property takes, in its own units.

With an input range of 0 to 360 and an output range of 0 to 100, a full turn of the source sweeps the property from 0 to 100.

Audio level

Sets the property from how loud an audio clip is: a speaker that pumps with the bass, bars that jump with the voice, a logo that pulses on the beat.

Source: The audio clip to listen to. It hears the clip after its level and fades, before mute and pan, so a muted clip still drives.
Band: All, or part of the sound: Low (below 200 Hz), Mid (200 to 2,000 Hz) or High (above 2,000 Hz).
Input range (dBFS): The loudness that maps to the output's ends. Starts at −60 to 0: silence to full scale.
Output range: The values the property takes for those loudnesses, in its own units.
Smoothing: How long the level takes to follow the sound, in seconds. Starts at 0.1. More smoothing is calmer; less is snappier.

Reset on the card puts the input range and smoothing back to where they started.

Follower and Pendulum

Two more drivers work on an effect's own controls, listed under Drive with after the others. Each follows another numeric effect property as its source.

Follower: Follows the source with a delay. Response sets how long it takes to catch up, in seconds, and Gain and Offset scale and shift the result.
Pendulum: Swings like a pendulum pushed by the source. Length, Gravity and Damping ratio shape the swing, and Gain and Offset scale and shift it.

Both have a Start time. On a property that isn't an effect control they're greyed out with "Only an effect parameter takes this driver."

Change or remove a driver

Edit driver in the row's menu opens the card again.
The settings are properties of the driver, so you can key them like anything else. Key a Wave's Amplitude from 0 up to fade the wave in.
Reset on the card returns every setting to its starting value.
Remove driver, on the card or in the row's menu, stops the property following the driver. It keeps its own value and keys.

Drivers.