The function of a buffer, if I'm not mistaken, is to provide a high input impedance to the source (the pickup, in this case) and a low output impedance to the load (the cable & whatever the cable's plugged into) The reason for using a buffer makes sense if you meditate on ohm's law long enough to understand some of it's ramifications. More on this later, unless I get sidetracked.
Gauss is the unit of measure for the strength of a magnetic field, so a low gauss magnet is... you guessed it, a weak magnet. Why use a weak magnet? Well, the stronger the magnetic field is where the strings intercect it, the more it will affect the vibration of the strings. In other words, weak magnets don't suck heap big tone. I think this is often referred to as "magnetic drag" They (weak magnets) also don't produce as strong a magnetic field where the coil intercepts it, which makes for a lower amplitude signal in the coil.
This goes hand in hand with their low impedance coils, which presumably have relatively few windings, relatively thick wire, or (most likely) a combination of the two. This has the effect of (correct me someone, if I'm wrong here) creating a coil with lower inductance and capacitance, which decreases AC impedance, broadening the frequency response in both the bass and treble directions. It also makes for fewer winding to intercept our (low gauss) magnetic field which makes for... you guess it, a lower amplitude signal in the coil.
Combine these effects and you get a pickup that has very little effect on the strings, a super wide (crisp sounding with powerful lows) frequency response, but jack squat for output level (with diminished ability to drive an instrument cable and subsequent amplifier stage.)
(is it the Heisenberg uncertainty principle that implies that you cannot measure anything without affecting it? These kinds of pickups try to interfere with the strings as little as possible - to have the smallest possible "observer footprint" note: some (many) people like the "warm" vintage sound - less highs, more mids, but if you're Kirk Hammett... bring on the wide band)
Oh, yeah... low output level...
What to do, what to do?
Oh, yeah, buffers.
Don't pay any attention to what I'm about to say - I really don't know what I'm talking about.
The buffer, in this case, is a way to protect the sensitive operation of the pickup from the great unknown of whatever it is you plug into your jack. If they didn't buffer the pickup and you plugged it into a 50' unshielded cable that went to a fuzz face, the results would not be pretty, ruining all their hard work. Placing a buffering circuit in there gives the pickup a consistent, known load that their engineers can count on when coming up with really sensitive, low impact pickups.
So: my guess is that they use a buffering scheme to reduce loading on the pickups (to preserve that extra tone they worked so hard to design into them) and possibly (probably?)apply gain to get the singal level up to standard pickup levels. They may or may not combine these in one stage...
A jfet (junction field effect transistor)in common source mode with a bit of gain and plenty of NFB would, if I'm not mistaken (but I usually am ; ) provide a nice high input impedance and enough gain, along with a low enough output impedance to achieve their goals
They might use an op amp
They probably use an op amp
Hopefully they use a *good* op amp but an ad797 is too much to hope for. I'd settle for an ne5532
Or they might use a BJT (bipolar junction transistor) as an emitter follower(re: buffer stage), followed by one hooked up in common emmiter (re: gain stage), follwed by *another* emitter follower... (this is unlikely)
The list goes on... a darlignton pair, a single BJT in common source...
Depending on the exact specs of their pickup and exactly what they wanted to achieve, they would design an appropriate buffering circuit.
In other words, I have NO idea what is in that buffer circuit, but I know what it's there for, and it's probably *really simple*
Now I'm just TOTALLY rambling... trying to puzzle this stuff out myself as I explain it.
Honestly, just wait for David Schwab to come along and correct me - he actually knows what he's talking about. And check out his web site - he makes pickups that imho are voiced better than EMGs (you can listen to sound samples)
Hope I created more questions for you than I answered ; )
(that's when you know you're learning and thinking)