http://mthtrains.com/content/40-4002
Bob,
Yes, it plugs into the transformer. It originally can with its own remote, but when DCS came out, the DCS remote was programmed to use it also.
Using this allows you to use the remote to run conventional engines with FIXED channels using the DCS remote. It actually controls the transformer remotely. I have my Variable channels set to FIXED and I can run all the TIU channels conventionally with this system. I think my Post-War engines actually perform better using this system.