Summary: A backward roll uses the index finger to start on string B. Learn how to pick backward rolls on a banjo in this free video lesson from a professional banjo player.
"The next roll I'm going to show you is the backward roll. It's not exactly the opposite of the forward roll but it does have some differences. This one starts on the index finger on the B string. Moves to the middle finger on the D string and then to the thumb on the high G. You do two sets of three and then one set of two. All together it sounds something like this. Now you hear the G string popping in and out. When you do this really fast that's what gives you that certain sound of the banjo in blue grass music."
eHow Article: Banjo Picking: Backward Roll