I noticed that players sometimes glide fretting fingers lightly up the string (toward the bridge) after playing an open string? Why do they do that and what sound change are they trying to create? Is it a quick but gradual mute? Thanks.
Yeah, I assume the OP is referring to a glissando. I do it all the time. Why? Because it is cool. Sometimes there is a reason like moving an octave (for example moving from the E on the 7th fret to the E on the 19th fret), but more often than not it is more of a transition. It is kind of like a wipe in cinema where there's one scene then a black wipe and then another scene.
Technically, I don't think you can glissando on an open string. You could hammer it down, then glissando off that fretted note. I have heard a player sort of glissando an open string by quickly turning the tuning knob. But that isn't the question. I've seen players move their left hand quickly up the string and pull off a harmonic tone. Maybe that's what's being heard here.
It doesn't look like he's doing anything but moving his arm for affect. I could be wrong, so why not ask the man himself. Andrew periodically contributes to a thread of his own
I was wondering if you were referring to Andrew's playing. Other people have asked him the same question on his website, and he replied that it is just an affectation and doesn't affect the sound at all.