Hi ScottYee

The organ has no dynamics of its own, so think of the pedal as adding dynamic expression to the organ. (Most electronic organs have expression pedals not volume pedals, as while they do the same, the expression name indicates how they should be used)

Acoustic instruments:
Are different in that you have to think what the real instrument can do, as an example:
You have a pre-set that contains 2 layers, (1 Piano and 2 Strings) now by default the expression pedal will alter the volume of both simultaneously, however a piano cannot do this and so it sounds unnatural, therefore turn the pedal off for the piano and leave it on just for the strings, now when you play you can bring the strings in and out with your expression pedal at any level you want, just like in a real Piano and String orchestra.

Orchestral:
Not all instruments can vary their volume by the same amount, (A clarinet does not have the same volume range as the string section) and by setting the expression pedal to have less effect on one voice (Clarinet) compared to another, (Strings) you get a much more realistic representation.

Mixing:
Set the brass section as normal, but set the strings in reverse, (Maximum volume of the strings occur with the volume down) you can now switch between the brass and string section (Or merge the 2 together) by moving the expression pedal forward and back. (Just like real bands do)

Electronic/synth:
Whatever you want to do (Just remember what the synth you are emulating can do)

I hope the above gives you some ideas, however the best way is to listen to real music played by real people and see how the different instruments interact, then set up your expression pedal to do the same.

Bill