Do you know what a lathe is? If not, I'll explain it the best I can without a picture. There is a hub that you screw the rotor on and a motor spins the rotor. Once the rotor is mounted tight, the runout is measured with a caliper gauge (like you used in physics or chemistry class). The runout is how much of the rotor is left, and there is a factory set minimum for the disc to work safely. Once you have determined the runout is within spec (runout number are listed in any Helms, Chiltons, or Honda factory service manual for the car). You then set the lathe to just below what you measured with your calipers. The book will list original thickness (mine was .910 inches, .830 minimum thickness). So, say you measured .860 inches with your caliper, you set the lathe to .850 inches. There are two diamond-cut needle cutters (one on each side of the rotor) that will start at the center of the disc and work out as the disc spins. This process "trues" the surface, removing rust, gouges, dirt, and any uneveness. Makes the surface smooth and reuseable again. The problem with trying to use slotted or drilled rotors is, if you think of the cutting in action, once those needles get to the slots or holes, they will do serious damage to the rotor, or worse, break the cutting needles or adjustment mechanism on the lathe. A good Moorse brake lathe costs about $2,000.00. Just can't do anything but smooth surfaced, solid rotors or drums on a lathe.