Those little rectangular pieces of metal with a hole through which the operating line passes on a drapery traveler track. As the curtain is opened, the backpans temporarily bind on the operating line and cause the curtain to stack beginning on the offstage side. Lesser quality tracks, without backpans, stack from the center outward, which doesn't look as nice and makes them harder to pull. The backpan (also called a backpack) is the little plate on the right. Photo courtesy of Mutual Hardware.