Phase Switching
All sound arrives as waves, and that means a series of alternating crests and troughs.  Even though a guitar picks up a single set of strings moving back and forth, most guitars have more than one pickup, so those waves can be in phase with one another or out of phase.

Why this matters is because when two signals combine, they do so either constructively (i.e., they cooperate and add together) or destructively (i.e., they cancel one another out and thereby diminish the output).

So how does it work?
This is roughly what the signal from a single pickup 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 pickups 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 pickups).  One wave is going up while the other is going down.  It's a lot like two people not coordinating when to push vs. when to pull.
.How's it sound?
Any two pickups that normally sound full and rich when combined will yield a thinner, more shrill sound.  Why would you want that?  It's great for reggae or funk where you need a bright sound.  Also, this will cut through a lot of effects or distortion that would otherwise make your tone too muddy.

The other just as noticable effect of an out of phase combination is that the volume drops.  This subtraction/cancellation is removing part of the signal, so your output is decreased by however much is removed.  In spite of this, a phase switch can produce the most radical alteration of your tone of anything described on this site.

So how many phase switches do you need?
Brian May from Queen has a phase switch for each pickup on his signature guitar, but this is more for convenience than necessity.  See, phase reversal is a relative thing.  If you reverse the phase of two pickups in use at the same time, you'd just have them back in phase with one another.  As a result, you really only need one less than the total number of pickups on a guitar.  For example, if you have a standard Les Paul, you only need one phase switch for the two pickups (unless you're going to throw in coil taps... but that's a whole other story).  Similarly, if you have a Strat, then two phase switches would be adequate to achieve every possible phase combination.  Remember, if you reverse the phase of two pickups, then they're right 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 pickup.  That's the pickup that is used in combination with the other two on a stock Stratocaster.  If you do a pickup add-in or a parallel blender to get new, non-stock combinations, then, yeah, you will get the neck and bridge pickups by themselves and need another phase switch associated with one of them, 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.  Start with one phase switch and work from there.
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