mccarty pickups split

I'm dabbling in the market/considering a SAS with the 3-way toggle & push/pull tone pot. I understand how the push/pull pot brings in the middle vintage-rail pickup, however I'm confused a bit by which HB also gets tapped in the process: Moore's web page, here, shows that for the ~roughly...

Hey folks - question for the gurus here. My impression is that PRS uses a split/tap scheme to allow part of the "deactivated" coil to still contribute some when split. This allows a stronger "single-coil" sound which I like. Is this done in the guitar wiring or the PRS pickups themselves? So...