The SPC or "Strat Presence Control" is often called the fat control. The original intent of the SPC was to turn the tone of a single-coil pickup into a dual-coil. This is accomplished by boosting the gain in a broad mid-band fashion and reducing the highs. The result is a fatter, louder tone from any single coil pickup, but you can also use it on a humbucking to give you a high output, really fat tone that's great for slide guitar.