From the Shergold website:
"The pickup is in effect two mini-humbuckers in the same casing. The stereo effect is derived by putting the pickup coils under the first and second strings on one channel and the third and fourth the other half of the pickup.
The main drawback with this arrangement is that in mono mode, the two halves of the pickup are connected in parallel, giving very low output.
Common owner alterations to the electronics are to add a toggle switch to flip between parallel and series connection of the windings, and a second switch to bypass the tone and volume network".
Hope that helps.