Side chains are used for two different techniques ... One is frequency dependant compression ... by inserting an EQ in the sidechain loop you can make the detector more sensitive to particular frequencies ... this turns the compressor in to a kind of "cut only" eq.

The other technique is called "ducking" or "keying". By triggering the compressor detector with the signal from another source, you may control the dynamics of the channel being compressed ... example .. kick drum and bass ... the bass is masking the attack of the kick drum ... so by "ducking" the bass with a comp being triggered by the kick drum, when the kick drum hits the bass is compressed a little each time the kick strikes ... this brings the bass level down for a very short time so you can hear the attack of the kick but the sustain of the bass.

amazing

kurt-

thanks for clearing it up for me!... the bass/kick method sounds really interesting; any other wide uses for 'ducking' on a standard session?

Sure ... this is not just for kick and bass ... it can be used for any elements that are competing for "space" in the mix ...

Radio talk shows do this all the time ... the hosts mic will duck the caller so the host can talk over them if they wish ...