A down shift mechanism for incorporation in a device such as an assembly tool includes a wrap spring which locks and releases a planetary gear speed reduction assembly to provide shifts between two drive ratios in response to torque throughput. The wrap spring is received upon a drive drum within a cylindrical housing. The ends of the wrap spring are engaged by a pair of relatively rotationally adjustable and fixable spring engaging lugs which adjust the preload of the wrap spring and thus the torque level at which the spring unwraps and lifts off the drive drum and thus the torque level at which the shift occurs. The wrap spring housing is coupled to the ring gear of the planetary gear assembly. When the torque applied to the wrap spring exceeds the preload, the spring unwraps, unlocking the ring gear and planetary gear assembly and downshifting to the speed reduction determined by the planetary gear assembly. An alternate embodiment of the downshift mechanism is also presented.