the mini flex is actually a microphone..
so you need to be mindful of playing with
a speaker pointing at you...and stay behind
the speakers to prevent feedback. The new
ones have the ability to get a stereo sound
with two mics...one near the sound hole and one
that you postion near the bridge...inside the
guitar... they require a AA battery which should
be changed or checked when you change strings.

these are invented and manufactured by a luthier
who knows guitars.