I recently picked up my first fretless. A 1985 MIJ Fender Jazz Special. I'm a little ham-fisted right now on it and am trying to get subtle vibrato. I can't seem to get much out of it by rolling my finger back and forth. Sliding it seems to be the only way I can get noticeable vibrato. But then I tend to overdo it. I realize I need more practice but, is there a trick to rolling your finger or is subtle sliding it the way to get decent vibrato? Do I want to let just the fingertip roll while it sort of grips the string, or do I want a lot of wrist action? Excuse me if this is a terribly simple question.