A sidechain send allows you to process the signal that will be driving the gain reduction without processing the signal that you will be hearing. So, in your case, you could feed your signal into the regular input of the compressor, take it out of the sidechain jack to an eq and emphasize the things you want the compressor to respond to, take the output of the eq back to the sidechain return and the compressor will respond to the signal that you've tweaked instead of the un-eqed signal. Very useful for keeping a compressor from responding to low end.
All duckers work that way, too. Split your "more important" signal, feed one part to the console and the other to the sidechain return, feed your "less important" signal to the compressor input. When the "more important" signal happens it will cause gain reduction to the "less important" signal, getting it out of the way.
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