Marion - I see you've asked the question twice about "...picking with thumb and index finger" - I may be wrong, but what I think they're talking about is picking a series of single melody notes, typical of flatpicking, but using the thumb and first finger and alternating them.

With this style you can pick nearly as fast as a flatpicker, but it often has a more powerful, in-control feeling for your hand. I've heard some banjo players who use this style say they feel like they can "rip the strings off the neck".

It's also seems easier to find the melody with the thumb and finger - why, I don't know. There is a slight difference in sound, however, between true flatpicking and the alternating thumb/finger.