JoolTool Ninja See-Thru Backpad, 3" Diameter (Pack of 1) At the core of our innovative sharpening system are our patented Ninja See-Thru discs, often referred to as backpads The combination of straight and curved slots within the disk allow the operator to sharpen, grind or polish with a crisp and clear view of the work piece The top has a molded fin structure that directs airflow onto the workpiece, keeping it cool The bottom is a perfectly trued, flat surface onto which our 3M NINJA abrasives adhere The backpads are easily mounted onto the JoolTool system by simply twisting on and off the spindle; they are self-threading and do not require any mounting hardware