This EMG potentiometer allows you to add even more tonal versatility to your strat by installing a "presence" control. More bite, and more rounded sound compared to the normal tone of a single coil pickup.
Here's what EMG say about the SPC Strat Presence Control Potentiometer
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.
In the box
Output Jack and battery clip set