An automatic shaft lock is incorporated into the transmission or drive components of a power driving tool of the type commonly used to tighten or loosen threaded fasteners, for example. The automatic shaft lock operates to prevent an externally-applied rotational back-force or back-torque, such as results from use of the tool to manually tighten or loosen a fastener, from being transmitted all the way through the drive components to the tool's motor or armature shaft. The shaft lock also effectively reduces the amount of back-torque, functioning automatically in either rotational direction, due to being disposed at an intermediate location in the drive train, between an intermediate gear enmeshed for driving rotation with the tool's armature shaft and an output gear enmeshed in a driving relationship with the tool's output shaft.