"The sweepers themselves are responsible for judging the weight of the rock and ensuring the length of travel is correct." When a rock is delivered, it is important that there be two players following the rock so that they are ready to sweep its path if needed. Sweeping is done for two reasons: to make the rock travel farther, and to make the rock travel straighter (curl less). When sweeping, pressure and speed of the brush head are key to slightly melting the pebbled ice in the path of the rock. One of the interesting strategy aspects of curling is knowing when to sweep. When swept, a rock will always travel both farther and straighter. In some situations, one of the two is often not desirable (for example, a rock may have too much weight, but needs sweeping to prevent curling into a guard), and the team must decide which is better: getting by the guard but traveling too far, or hitting the guard. Much of the yelling that goes on during a curling game is the skip calling the line of the shot. The skip evaluates the path of the rock and calls to the sweepers to sweep as necessary to hold the rock straight. The sweepers themselves are responsible for judging the weight of the rock and ensuring the length of travel is correct. Usually, the two sweepers will be on opposite sides of the rock's path. Grip of the broom is vital - one hand grips the top (non-brush end) of the handle while the other grips the handle close to the head of the broom so that as much pressure as needed may be applied while sweeping, though the precise amount of pressure may vary from relatively light brushing to maximum-pressure scrubbing. It is important to never to touch the rock while sweeping (a rock touched by a sweeper is burned, and the opposing skip may opt to have the rock removed from play). Sweeping can be done anywhere on the ice up to the "tee-line", as long as it is only for your own team's rock. Once your team's rock crosses the tee-line, only one player may sweep it. Additionally, when an opposing rock crosses the tee-line, one player from your team is allowed to sweep it. This is the only case that a rock may be swept by an opposing team member. In international rules, this player must be the skip, or if the skip is throwing, then the third. http://en.wikipedia.org/wiki/Curling#Brooms_.28or_brushes.29