Series Vs Parallel Switching
Usually when multiple pickups are installed in a guitar, they are connected to operate in parallel. This means that the signal passes through the pickup from ground and then on to the tone circuit, volume control, or straight to the pickup selector.
Even when two pickups are on at once, you are hearing the sound of them working independently.
When you wire two pickups in series, the signal passes through one pickup, straight into another pickup... and then back to the rest of the circuit. When wired correctly, these can be hum cancelling. You can have a "humbucker" with two single coils.
When you use this , it can open up a whole new palate of sounds.
Watch the Video below to hear what it sounds like, and a couple different applications of the concept.