I was out at a show the other night and saw a guitarist use a little device which I've seen before but don't know the name of and would like to implement it in a couple of my songs. It's something which you hold over the fret and maybe lightly press down and it sort of slowly draws out that note. It looks like it is slowly vibrating on it to create a very high attack gradual fade in note. If someone could clue me into what this gadget is called, I'd appreciate it.