Think of it like a tube preamp, Mesa Boogie's Quad Preamp was "loose" on the low end for that huffy sounding mute, and tightened up on the high end. The Peavey Rockmaster preamp was the opposite, really tight lows, highs were really loose, made tapping nice. I would combine them sometimes, but really just wanted to loosten up the Rockmaster preamp so the lows were huffy too, then it would've been perfect. There was a mod recommended to me to do this, but I can't read a schematic.
Okay now that we're thinking along those lines, the resonation loostens up the lows and highs all together so everything is perfectly even. There's a different feel to it, especially if you are muting the highs. You get something that works well for legato and tapping, and huffy muted lows, harmonics breathe...
The only drawback is what made the Mesa Boogie preamp sections is that those highs tighten right up when you need them too the most and you get a tight pick attack and harmonic. So this is more of an old school metal kind of sound than anything, and not the Dream Theater/Petrucci sound, or Metallica, but is quite useful...seems to help when rolling off the volume also so maybe it's what you're looking for if you're needing to use one amp/preset for your gig or whatever.
The original idea was just to improve the live sound when not using cabinet simulation direct if I remember right. But one time I was playing around and left a cable in the Output to Amp and it felt different, so I kept playing with it out and in again and indeed the resonation detects the cable and loostens up the signal and utilizes a capacitor (or bypasses one), but it's just like a mod that you can turn on and off as you see fit
Just enable it in settings, and plug a gutar cable into the front Output to Amp and see what you think. Seems to make things easier that I like to play, so I've been using it a lot lately again with my old presets that I must've dialed in with it on I'm thinking. I was going to post a reamp comparison, but I liked the way this sounded better. Maybe I'll do one, but I know for a fact in a reamp I did once, it worked GREAT in a mix and I blended a boogie and classic peavey-ish sounding amp/preset with little gain on the 4 rhythm tracks.
https://soundcloud.com/tltd/rose-of-shryn-reamp-through