In a way, it does work like a semi with a dual range trans, but not in how the controls work. The "main" transmission (the overdrive takes the place of the tailshaft rather than being separate) works just like the standard 3 speed. The stock shifter mounts right up. The overdrive unit is controlled in part mechanically (Freewheel clutch/overdrive lockout cable), and partially electrically (overdrive negagement/kickdown solenoid).
Here's a full explaination on how it works.
