This thread has given me a better understaning (I think) of sidechaining because of all the searching I have now done (in regards to my RNC).
I think one problem is its sometimes hard to explain in writing, with inputs, outputs, internal, external audio etc..
So heres what I have gathered in regards to RNC.
You have 2 audio signal paths (lets forget its stereo ftm).
The first (#1) is what you hear, audio in to compressed audio out.
The second (#2(a copy of audio in)) is used as the control source that the compressor uses to do its thing on the audio #1.
Now the sidechain, when you plug in a TRS conector/cable, it breaks the conection of audio #2 (control), or another way of thinking is it sends that control audio out (to be EQ'd, filtered or what ev's) and also receives it back again on the one conector, hence an insert cable is used. So its completly seperate to what you hear on the audio outputs other than it is used to "shape " the audio #1.
So in theory (still havn't tried this) you could for example have a synth pad going into the main inputs and have a different sound (kick) pluged into the sidechain's in, to get your Daft pump. And not hear the kick (because its actually a control source).
So I dont get what this "key" thing is.
Does it simply mean that, as like in the above example, except that the kick will be mixed internaly with the compresed audio output, so you do hear the kick and the now compresed synth pad ?