The first three concentric controls are individual volume and tone for each pickup. Next in the circuit is the solitary one, a Turnstyle switch, and the last concentric control (closest to the jack) is a master volume and tone.
The Turnstyle is a 6-position tone preset switch, not a pickup selector switch. (It can be used as a pickup selector, but it can do far more than that. It all depends on what options you want.)
The way it works is that each individual pickup tone control has a small capacitor to take the edge off the highs. It's not nearly enough to make the sound dark and flat. And each of the individual volume controls are no-load, so when they're wide open, they take themselves out of the circuit entirely. The three pickup controls then feed into the Turnstyle Switch. Position 1 gives me full manual control over all pickups. Positions 2-6 are tone presets.
Michael at Turnstyle is able to take whatever pickup signals are provided, and electronically tweak them to get the desired tone. Obviously, the better signals he gets from the pickups, the better the end result can be. He was blown away by the tone of these pickups. Anyway, back on topic -- the tone presets I have are Precision Bass, Jazz Bass, Rickenbacker, Thunderbird, and Overdrive.
From there, everything goes through the master controls. Again, the volume is no-load, and the tone control is limited.
Note: I hate the tone controls on most basses. After you turn the knob about 1/2 - 3/4 of a turn, the sound drops off a cliff. I have never used that part of the tone control's range, and I don't know anybody else who has either. I didn't want that on my bass. So a full turn of my master tone control accomplishes about what half a turn would accomplish on most instruments. Combine that with the minor tone adjustments available on each pickup, and I have a rather large tonal range available to me, and all of it is useful.
Note: This is all passive circuitry. People like their preamps, and I don't begrudge anybody the right to like what they like. But they aren't necessary for good tone.
Note: Apparently, it is normally a bad idea to have consecutive tone controls in the same circuit. I say "apparently" because I have to take people's word on something like this, and people whose electronics backgrounds I trust are rather consistent about this. However, Michael developed a proprietary workaround. I don't know what he did exactly, but he made it work.