The EM2000 is a microprocessor based control system and how it "works" depends on its programming, which differs between different models of locomotive in which it is used. Depending on level of discussion the answer to your question could require writing a book. I would suggest you register at eBay and look for the operators manuals of locomotives which use this system. Use the search string "EMD MANUAL" in looking for them. They often show up there and can usually be obtained in the price range of US $25.