It's a pre-eq-shapeMy best guess is that the switch probably does something related to the light compression added to the clean settings, either entire switching it off, or perhaps stepping up the compression rate, or threshold, or something like that.
This is of course entirely a guess.
Just seeing the date this was posted, I suppose it should have been answered by now, so what does it do?
I didn't even realize there was a Mk II version.