The sidechain input can be thought of as a trigger input. If you want sound to be compressed (ducked) whenever a kick plays, for example, you can send a copy of the kick signal into the sidechain input to trigger the compressor to act on the main signal flowing through it.
To make a copy of the trigger signal, use a splitter, that way the original version of it can still end up in a centroid and be heard.
hook up whatever device you want through the your compressing device whether its the Compressor or Gravity device, and then hook up another device into the sidechain input and then it will put sidechain on the sound going through the first input
Adjust the threshold, ratio, and attack/release to your liking as well
Thanks!