An impact driver is a manual tool that delivers a strong, sudden rotational and downward force. In conjunction with toughened screwdriver bits and sockets they are often used by hitting a hammer onto the driver to loosen larger screws (bolts) and nuts that are corrosively "frozen", rusted or over-torqued. The direction can also be reversed for situations where screws have to be tightened with torque greater than a screwdriver can reasonably provide.
