In switch positions 2 and 4 on traditional Fender guitar circuits, pickups are combined in parallel for that fab quacky 'out of phase' sound – but those single coils can also be combined in series, like the coils in a PAF-style humbucker.
Since all but the earliest original Strats already have five-way switches, most people fit a push-pull pot if they want to add series switching to the stock settings. Push-pull pots are readily available these days, so you just have to substitute one of your volume or tone pots. Some players might also be interested in modding their Telecasters for series wiring using a push-pull switch, so in both cases just follow the wiring diagrams (see Fig 5 and Fig 6).
With the push-pull switch disengaged your Strat will function like a regular Strat, but when it's on you'll get three interesting new options. The middle position combines the middle and neck pickup in series for a very rocky tone, while the back position gives you the bridge and neck pickups in series. Again the sound is louder, raunchier and darker, but it's phasier and more diffused than a bridge humbucker on its own. However, position four is the best of all because the bridge and mid are in parallel with each other, and in series with the neck. It's like an out-of-phase Strat on steroids.
Series mode won't buck hum like a true humbucker unless your pickups have reverse windings and polarity, but it gives a louder, darker, rockier tone with many trademark humbucker characteristics. A simultaneous volume increase and treble roll-off is, ipso facto, a bass and midrange boost. It's great for powerchords and solos, but you can still hear the character of your guitar.
Four-Way switch for Telecasters
You can swap the regular tone pot for a push-pull pot on Teles, too. With the pot switch pressed in your Teles will operate like a regular Tele, but you can disengage the three-way switch to go straight from any pickup setting to parallel mode and back again (see Fig 7).
However, you could also employ a four-way switch instead of a push-pull. This modification has become so popular that even Fender is now offering it on some standard models, such as the Baja Tele. The four-way switch is a direct replacement for the stock three-way, and it allows an extra pickup combination without requiring a push-pull switch.
It's a simple mod to do but, as with the push-pull method, you need to ground the neck pickup cover separately. Start by snipping the connection between the cover and the negative lead of the neck pickup. Solder another wire to the cover, and solder the other end to a convenient ground connection on the underside of the control plate. Once you've done that, just follow the diagram. The series tone is so good, it's hard to imagine that anybody would choose to reverse this mod.