What's that knob do? ...No, the other one, to the left and further up from the switch thingy, not far from the corner of the unit with the digital metering near the back... what happens when you turn that knob?
As a professional audio engineer, the most common question I get from folks wandering by FOH when I'm mixing live sound is "Do you really know what all of those knobs do?" Seriously, I've probably gotten that question at least once a week for the last ~30 years.
I once pointed to some completely undifferentiated red knob in a sea of red knobs and said "That one right there... I have no idea what that one does."
That was way more gratifying than my usual response, which is it to smile wryly and say "Yeah, that's my job."