in poor words sidechain is a sort of gate fx+compression fx applied to a signal (sound): in other words you make this gate fx with a compressor fx imputing an external signal into the compressor to "drive" the gate of the other signal (sound); the first signal is not heard; you have like this a sort of "breath" effect.
others talk about this as "duck" (or ducking) fx.
there were an interesting thread about sidechain with softs made by HD (hurlingdervish) on the forums...

i've tried something weird with my joemeek 3q, the zoom st224 and the xio25.
listen.
i reasoned considering the st224(but it could have been sp303/404/505 etc) as an fx box, with send(the in)and return(the out), just like the 3q insert in accept external fx with this in.
so i put the insert cable in the 3q's insert jack and then i put the send/return jacks back to the output and the input of the st224 (opening the source mix-is like the ext source on the sps-), i obtained a sidechain fx, but the sound comin' from the insert cable (the st224 sound in this case) is heard...
however is better then nothing...
im still waiting for the 3630 comin'...