Drone

In November Rain live by Guns n Roses Slash puts his hand over the pickup or strings and makes a sustained note drone or hollow.
What is he doing??!?!

Slasha will know... hopefully

Haven't seen the video you're talking about, but if the origianly played note turned out to a higher pitched one (usualy one octave higher) , then it's definately a "Tap Harmonic". I know that Slash use it a lot.

The easiest way to do it, is to play a single note, count 12 fretts higher from that note and tap right above the fret wire (don't touch the fret board, just like a light slap).

He pics the 19 fret then puts his hand over the bridge pick up

If it's a harmonic, he could just be touching the string, instead of tapping a note 12frets up you can just lightly touch the string 12 frets higher and you'll get the harmonic to come out, if Slash is playing at the 19th fret, all it takes is a little experimentation to find the right spot to touch, just because their are no frets underneath it makes no difference, start hunting halfway between the bridge and the 19th fret and maybe you'll get lucky. Having said that, there is another possibility, Slash could just be muting some strings with his hand, and letting the amp feedback the sustained note, most tube amps will sound the harmonic when they start to feedback.........