The series-parallel-tap switch allows a humbucker pickup to have its internal wiring switched so that the 2 coils are in series (standard configuration), parallel wiring or with one coil removed.
Why would you want it?
This gives you a parallel wiring option in addition to the normal coil cut/tap. With both coils in parallel you maintain the humbucking quality of the pickup but with a single coil sound.
The coil tap option will reduce the output from the pickup considerably, with this setting you effectively have a single coil. The parallel option combines both coils but in parallel wiring, you still lose output level but you maintain its humbucking properties.
What do I need to do it?
A humbucker with 4 conductors (it is very important you are clear on the manufacturers colour codes)
A DPDT on/on/on switch (this is a special type of DPDT with 3 positions, see below)
A bit of wire and soldering gear
Note: You can still use a standard DPDT switch or an on-off-on DPDT for this mod but you wont have the coil tap/cut option in the middle position.
How does it work?
The objective here is to wire the switch so that position 1 gives parallel wiring, the middle position grounds 1 of the coils (tapped) and position 2 gives the standard humbucker sound (series). It is essential you have the correct type of DPDT switch or this will not work. It will be labelled as an on-on-on switch, bear in mind there are different types of these on-on-on switches you will need the one that works as follows.
If in doubt conductor B (green) in the first diagram (white for SD pickup) is the ground connector for coil 1 (you can test this by checking for continuity with the hot connector). Conductor D (white) in the first diagram (red for SD pickup) is the hot connector for coil 2 (you can test this by checking for continuity with the ground connector).
This picture shows this wiring (irongear colours) on the Anderstrat guitar.