 A typical transmission is designed with "A" and "B" gears, so when the engine spins, the outputs shaft spins the "opposite" direction, which drives the vehicle "forward".A reverse idler gear setup is actually typically an "A" and a "C" gear, which are not in contact with each other until a "B" gear is moved between them.Since the transmission is designed to move the car "forwards" when the output is spinning in the "opposite" direction from the input shaft, when added to the "B" idler gear, it forces the "C" gear to spin in the same direction as the "A" gear, and thus the input and output shafts are spinning in the same direction, which drives the car in reverse.