Aren't screwdrivers levers?
The whole process is actually the friction transform: friction power screwdriver rotate from the hand and the handle, transfer to the screw through the lever amplification, to overcome the friction screw in the screw hole of the screw rotation.
The same principle is the same, with the lever, the lever wrench is. When used a screwdriver, screw axis is the axis screwdriver as a fulcrum, the screw rod radius is a lever, a screwdriver knife (handle) radius is another arm, under normal circumstances, screw is less than the handle, the screwdriver is a labor-saving lever.
