This is a small unit which rests on the keyboard, which behaves like the drawbars on a tone-wheel organ. It transmits the settings via MIDI to the V3.
It makes life a lot easier when playing live, although not all parameters are available, and some are duplicated. It seems like there is a newer version available which is a bit more intelligent - my unit doesn't transfer the buttons' status when I re-transmit, only the drawbar levels.