You would perform this modification to affect the middle pickup of a guitar with 3 single coils (e.g., a Fender Strat).

The illustration shows how to put a phase reversing switch on a push-pull pot (I don't have the pot wired in this picture; it can serve any function).  You don't have to use a push-pull pot as any DPDT (double pole-double throw) switch will work.

The bottom configuration (the way it would be if you had the push-pull pushed) is the normal phase (what I call "in phase").  The top (pulled) configuration produces a phase cancellation between coils.

The (-) wire is the black one.  The (+) is usually red, sometimes white or yellow.  The wiring running to the 5-way switch goes to the same place on the switch as the middle pickup was originally connected (I didn't bother drawing this as different guitars have different types of switches).