These drives are often mistaken for one another due to their obvious similarities, as they both couple a spinning "rim" of sorts to accomplish the end goal, it can be confusing at times to discern between them.
The difference lies in the mechanical component coupling, whereas the rim drive couples the motor directly to the platter periphery by means of direct motor to rotating spindle contact, the idler drive couples the rotating motor spindle to a passive rubberized wheel which once engaged provides the rotational force and simultaneous secondary point of contact at the platter side which sends the platter into motion.
Chris, the VPI rim drive is decoupled from the twin motors and in addition, also acts as a flywheel.
