Phase Switching
This mod will take any two pick-ups that normally sound full and rich and turn it to a thin and shrill sound.  Why would you want that?  It's great for reggae or funk where you need a thin sound.  Also, this sound will cut through a lot of effects or distortion that would otherwise make your tone too muddy.

Incidentally, Brian May from Queen has a phase switch for each pick-up on his signature guitar (although this is a bit excessive, to be honest, as one is adequate).

So how does it work?
This is roughly what the signal from a pick-up looks like.  The x-axis is the time and the y-axis is the amplitude (volume) of the signal.
Now, when two coils are activated together, the signals you normally want to hear (those produced by the guitar strings) are added together, but if you wire the pick-ups out of phase with one another, they cancel out all of the sound produced by each except for the differences between them (the difference in the sound between the two pick-ups).

This subtraction/cancellation is why the volume drops when you have phase cancellation.

Phase switching with a 3-pick-up Strat-style guitar:
You would perform this modification to affect the middle pick-up 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.  The wiring running to the 5-way switch goes to the same place on the switch as the middle pick-up was originally connected (I didn't bother drawing this as different guitars have different types of switches).

Phase switching with a guitar with humbuckers:
Just follow the same scheme as above with the two leads from your pick-up.
Phase switching within a humbucking pickup:
This modification can be performed on one or several humbucking pickups within the same guitar.  The theory is the same, it is just that the phase switching is performed on what would be considered half of one pickup.  The sections below describe the wiring within individual humbuckers.

Series, out of phase
This scheme throws the two coils out of phase with one another.  There is less output produced than standard wiring because the pickups are effectively canceling one another out except for the differences in their respective sounds (which isn't much if the two are close to one another).
 

Side by side comparison

At left is the standard in-phase wiring.  At right, the bottom coil has been electrically reversed to be out of phase, but the connections are the same.

Parallel, out of phase
This is much the same as the above wiring scheme.  Once again the coils almost completely cancel one another out.  This is useful for more exotic applications, but nothing you would use all the time.

 
Side by side comparison

At left is parallel in-phase wiring.  At right, the bottom coil has been electrically reversed to be out of phase, but the connections are the same.
So how many phase switches do you need?
You really only need one less than the total number of pick-ups on a guitar.  For example, if you have a standard Les Paul, you only need one phase switch for the two pick-ups.  Similarly, if you ave a Strat, then two phase switches would be adequate for achieve every possible phase combination.  Remember, if you reverse the phase of two pick-ups, then they're righht back in phase with one another.

Now, as far as I'm concerned, if you have a Strat, then you would be perfectly fine to only install one phase switch for the middle pick-up.  That's the pick-up that is used in combination with the other two on a stock Stratocaster.  If you do a pick-up add-in or a parallel blender, then, yeah, you will get the neck and bridge pick-ups, but do you really want to go overboard?  Okay, I do a lot on this site, and even I haven't installed more than one phase switch on a guitar.  Save your time, energy, parts, labor, and control cavity space for more useful mods.  I think there are a few on this site that might work for you.

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