Heh Phil,
Its pretty straighforward depending on what you want to sidechain or duck.
Lets say you have a heavy/thick acoustic guitar section that is covering up the vocals when its at its hottest part and you dont want to automate the buss or track because, from then on, you will be adjusting volume envelopes or adjusting offset.
1. Insert the CA2A on your Electric guitar track or buss and leave the setting at default for now.
2. Go to your vocal channel or vocal Buss and click the + icon in the "SENDS" window under the FX bin.
3. Scroll down and click on CA-2A (Input 2) of your offending Guitar buss.
4. Play the section back and the vocal channel will now 'trigger' the CA-2A and start to compress the Guitar Buss. The settings for gain and reduction will control how much as will the Vocal Channel 'SEND' amount. Allthough, I usually leave this at the unity default 0.0db.
Very handy and much more subtle/surgical than other methods of dynamic control ( automation, clip gain, ect. )