The Variable Input Impedance works best with passive instruments (guitar/bass) and allows tone-sculpting with or without the fuzz circuit engaged. Lower settings yield softer dynamics/transients with a notable ‘rounded/vintage’ feel and tone. Higher settings gives a sharper high frequency response with a more direct/aggressive transient feel and more perceived gain. This makes for a great ’shaping’ control, where you can tune the pedal for its best response to the pickups you are using and how you want them to react/feel.