The hell is geater when this particular magnetic pickup (but I should be sure, all the cheap ones, too) placed on a classic guitar the amp detects the sound!!! (Watch it at the beggining of video - and later the simple explanation).
Sorry, I'm not watching 13 minutes of explanatory vid, but I've been using microphonic magnetic pickups for many years. Here's an example, on my old National reso:


This recording is from the amp, no direct acoustic content at all.