Sweeping is what makes curling a team sport.  To be an effective sweeper pay attention to the rock's five-inch running edge.  Any excessive movement of the broom outside of the rock's path is wasted energy.  Speed and pressure generates friction on the ice which allows the rock to glide further and/or a straighter path. Skips will tell you the line of the rock and the sweepers need to know the weight.  Sweepers need to get a feel for the ice, observe the distance the rock will travel, the speed, and the rate at which the rock is slowing down.  Clean the line of delivery before every shot, stay with the rock until it comes to a complete stop, and be prepared for alternate shot calls "Plan B"