A device that automatically actuates the reversing lever of a ratchet mechanism when the radial direction of the motor is changed. The auto-shift mechanism includes a worm gear that rotates with the motor, and a disc coupled to the worm gear that moves laterally as the worm gear rotates. Axial movement of the disc actuates a reversing lever of a ratchet mechanism by either pulling or pushing the reversing lever. A lost-motion mechanism can absorb an initial amount of torque so that the auto-shift mechanism can run its course prior to applying the torque to a fastener.