An impact driver is a tool that delivers a strong, sudden rotational force and forward thrust. The force can be delivered either by striking with a hammer in the case of manual impact drivers, or mechanically in the case of powered impact drivers.
It is often used by mechanics to loosen larger screws, bolts and nuts that are corrosively "frozen" or over-torqued.[1] The direction can also be reversed for situations where screws have to be tightened with torque greater than a screwdriver can reasonably provide.