Side-chain is a I/O on compressors it is not a function, what it does do is allow DIFFERENTIAL COMPRESSION.
This is actually different to Keying or ducking. Keying and ducking are what most of you guys mean when you talk about sidechains. Sidechain differntial compression is used to de-ess as well as kinda making it a band compressor, say boost low Frequencies with an EQ to signal from sidechain and return to side chain and it will compress hi Frequencies and not compress low Frequencies. can set up multiband compressors this way with multiple units but yeah we have computers now.
I use hardware compressors not software, but software is modelled on the hardware obviousliy. so same principles. If you guys understand what i mean you will greatly benefit IMHO.
My hardware compressors work counter intuitively, as you use 2 channles as seperate I/O to do KEYING and DUCKING. NOT sidechain...weird hey took me a while to figure it out even with manual,

must be all the crap i read on the internet
Example of how i key or duck - so channle A has kick, channle B has bassline, every kick triggers compressor and bassline is quieter so kick is louder. This technique is common in radio broadcasting as it lowers the music when you talk. leaving sidechain unused, sounds weird but thats how alot of hardware compressors work. So try it my way and you might be surprissed how well it works. I will post eample soon, but need to repatch