I've been doing some research, and here's a brief synopsis of how an AR works:
Basically, after you enter the code input (which, by the way, is a hexidecimal code), the Action Replay (or any cheat device, for that matter) will search through the game cartridge's hexidecimal code banks and replace the relevent code section with the code input (the cheat). This will affect the gameplay, sound or graphics in some way. That's about as technoid as I can get. Unfortunatly, I know of no Super Mario RPG Action Replay Pro codes, sorry. I'll keep you informed if I do.
Well, this is where the signature goes...