The hammering action in a Hammer Drill provides a short, rapid punch-thrust to pulverize relatively brittle material and provide quicker drilling with little effort. It is measured in BPM(blows per minute) or IPM(impacts per minute). Hammer drills are used for drilling holes in heavy materials such as masonry, concrete and stone. The heavy, hammering action helps to break up the masonry so that it can be removed by the drill bits flutes.  A good quality hammer drill will have a lever or switch that locks off the special "hammer clutch," so that the tool can be used as a  conventional drill for wood or metal work.