I ran across the idea (and the schematic) over on a page that seems not to exist anymore, and which was run by a guy named Mike Torchia. (Here’s the archive.org link, as long as that still exists.)
Anyway, I often have needed to switch quickly from a good drive to a lighter distortion and rolling down the volume knob on my guitar is a nusiance – never mind imprecise – and this thing essentially does the same thing with a stomp switch. Here’s the schematic (from Mike’s page):
The capacitor across the pot allows higher frequencies to continue making your sound sparkle instead of loosing all that treble, like when you turn down the volume knob on the guitar. I have mine turned down a lot to give a lighter drive instead of my normal distortion and it’s a little finicky to set, but I think I may have used a linear taper pot instead of log taper. Ues log tapers for audio, folks!