Most of the mag pups are acoustic modelers now. Read the adware. Strap one to a cinderblock, same as strapped to a Martin D45. Doesn't matter that most people think they're picking up the sound of their guitars, which they aren't, as long as they keep thinking that the makers will make sales. Piezo crystal generates electricity when vibrated. The myriad string frequencies acting upon it are then amplified at the preamp and sent out. The art of placement is one of the most important aspects of Piezo bugs. I have three placed inside one guitar. It's an array made by K&K and designed to separate the lows, mids and highs coming off the soundboard. They are attached to the underside of the bridge reinforcement plate inside the guitar, forward of the pins.
Straight (unplugged) acoustic bass and low mids are largely created by the chamber of the soundbox and come out of the soundhole. The higher the freqs, the more they are generated off of the top wood, which is why people have preferences for certain top woods. The Piezo bugs affixed to these varying wood species soundboards will impart wood-distinctive frequencies (cedar, mahogany, spruce, redwood, etc) into the Piezo which then gives a palette of sounds from those varying wood species. Mine are soundboard Piezo tranducers (SBT) in one guitar. Then there's the under saddle ribbon type of Piezo transducer (UST) that receives its frequency excitation through the saddle, picking up little of the soundboard modified frequencies sensed by the soundboard 'ducers. (SBTs). The SBT is highly susceptible to feedback from any old stray signal acting upon the soundboard (drum skin). The UST is less susceptible being somewhat isolated from that under the saddle.
In any of the above methods, the acoustic guitar becomes an electric guitar. The electric guitar mag pup senses the ferrous metal movement of the string and sends it out to the amp no differently than a similar device lashed up to an acoustic guitar. Conventional acceptance is that an acoustic remains an acoustic regardless of how its sound is captured and sent. Technically, though, if you wire it up it becomes transformed into an electric.
Then there's the condenser mic system some acoustic onboard systems employ that also use a Piezo (SBT or UST) with volume and pup source blending wheels. I have the LR Baggs condenser/UST in one guitar and when ambient noise calls up the feedback I blend in more mic that UST. The guitar mounting strictly an SBT array has to be placed carefully in a "mix" to prevent feedback. My amp has a notch filter to counter the feedback and it works pretty well.
Then there's the microphone sensing method many purists insist on using in an X-Y pattern at the 12 fret to get the sound out. Because this method generally does not alter the true sound of the guitar I'd argue that this is the only method that can be said to leave the guitar purely acoustic. I'm a whore. I choose whatever gets the sound out. What I won't do is use a pedal of any kind, or paint racing stripes on a Toyota Camry.
Edit to add -
http://village.melodyvine.com/forum/viewtopic.php?f=17&t=233