Hey, Kids: Rock And Roll -- In Tune This Time
As the strings are played, the Powertune processor compares their actual frequencies with the desired notes and sends instructions--tighten the string this much, loosen the string by that much--to tuning pegs equipped with strong, tiny servo motors mounted on the back of the guitar's head. Because onstage interference could potentially degrade a wireless signal, the system uses the strings themselves to send the signal.
For this reason, players typically tune between songs, rather than keeping the system active while they're playing--otherwise, the changing vibration of the strings being played and the electrical connection between the strings and metal frets could confuse the system.
No word on whether the tuner goes to eleven; could be a problem if you want to play "Stonehenge."