As per the Pete Jackson installation instructions:

“In normal operation, power is transmitted through the “POWER IDLER” gear (the bigger one) to the camshaft gear. The “REVERSE IDLER” (the smaller one) gear is rotating under no-load and must have sufficient clearance. The sole purpose of the “REVERSE IDLER” gear is to prevent the “POWER IDLER” gear from being disengaged whenever the crankshaft gear rotates backwards, as occurs each time the engine is switched off.”

And there you have it….