You can think of it as a knob that interacts or changes the “grit” of the Drive function. It will quiet things down if you’re rolling it back (which is actually turning it up) and not compensating with the drive and volume. Think of it more as a “character” knob, except one that doesn’t necessarily drastically cut or boost any frequencies. To me, the Sag is where the real magic is on this pedal. My personal settings are as follows: If my Sag is just below noon, my Drive is just above noon. That’s pretty much how I’ve left it since my first Barbershop.
One thing I’ve definitely noticed is what must be a slight variance in the tolerances of pots used across models, sometimes within the same run. All of my Barbershops behaved somewhat differently from the one prior with regards to the sensitivity of the Sag and Drive controls. So far, the Modele B to me is the most like the original big box version, which also happened to be my favorite. The one a few posts back that I sent out for repair came back with a Drive control that was WAY more sensitive than any other I owned before or after. Whatever it was FFC did to it wasn’t for me, but a guitarist I knew loved it so I sold it to him and bought another one (worth noting here how super cool FFC was about the repair, only charging me shipping to get it done). My second and third ME were all within the scope of what I’d consider “normal” function for a Barbershop. The Modele B reminds me most of the original, though I know on paper they’re all supposed to sound the same at the “stock” setting. The variances I’m talking about are subtle and probably something most people wouldn’t notice unless they’ve owned multiple examples...maybe not even then. Or it could all be in my head!