Bobbin work is when you use decorative threads or yarns( any thing too large to thread through the needle) in the bobbin and work from the wrong side of the quilt or fabric to embelish . Often very narrow ribbons are used , but almost all of the heavier or thicker fibers are hand wound on to the bobbin. Some bypass the bobbin tension or adjust the tension crew on the bobbin case to get the proper balance. I have a seperate bobbin case just for bobbin work , as too many adjustments in the bobbin case can get tricky whe going back to regular thread.