Serial Phaser Overview and Wiring Diagram
This is a fantastic alchemy of my two favorite switching options between two pickups - Series/Parallel and In/Out of Phase. The best part about this configuration is that it eliminates the most common problem with a series/parallel switching between two pickups and a selector switch - either a dead spot (some players like this because it acts like a mute/kill switch) - or - a duplicate toggle position. There's nothing more wasteful in my mind than a duplicate toggle positions. This configuration actually overrides the toggle so that regardless of whether you're in the bridge, middle, or neck position, pulling the volume pot puts the two pickups in series.
As with following any diagram it's important to take your time and make sure that you understand where you're making connections and why.