Sidechain
A sidechain input makes a compressor or filter listen to another signal. With nothing connected, it listens to the sound it is processing.
Sidechain input makes the detector listen to a different signal than the one it affects.
On a mixer channel the control is Sidechain input. Feed it from another channel. The usual patch is the kick into the bass compressor: each kick pushes the bass down, then the bass comes back. That dip is what the channel page calls ducking or pumping. Release sets how fast the bass returns. With nothing connected, the compressor listens to its own signal. Loud moments of a vocal turn that vocal down.
Gravity and the Compressor work the same way. An external sidechain replaces the main input as the trigger.
Autofilter can follow level with an envelope follower and use the result to open or close the filter, so the motion changes tone. Connect its sidechain input and that level comes from the other signal. A kick can then brighten or darken a pad on each hit.