If you've played a Fender Jazzmaster you know it's got a standard Volume/Tone knob setup with a 3-way switch for the two pickups--pretty standard stuff...but it also has that little black switch up above the neck pickup and those two mysterious sideways knobs.
I'll be honest. I'd played Jazzmasters but never owned one and whenever I got one in my hands I just had no idea what the extra controls were for. If you're like me, here's the deal: Assuming it's a vintage Jazzmaster, or the more recent Classic Player Jazzmaster Special made in Mexico, the switching will be the way the Jazzmaster was originally envisioned by Fender. That means the black slide switch above the pickups when UP position is in normal or "lead mode." In lead mode, the white 3-way pickup switch and Volume/Tone knobs down below work just like any normal guitar. The black switch in DOWN position engages the "rhythm circuit" which uses only the neck pickup and the two black controls for volume/tone, and it has a much warmer "jazzy" sound.
This rhythm circuit was thought to be much more useful back in the early days, as Fender was trying to make the Jazzmaster emulate popular archtops for jazz situations. However, the circuit fell out of favor through the years, prompting Fender to introduce many new incarnations of the Jazzmaster with alternate wiring setups.
Some modern players, such as Elvis Costello and others, led something of a revived interest in the traditional rhythm circuit, so it has survived in certain models such as the Classic Player Jazzmaster. Now consider yourself schooled on Jazzmaster switching. Next time, we'll tackle the switching on the Fender Jaguar. In case you're wondering, that picture at the top is the current Jazzmaster we have for sale.