When the sweepers are sweeping for line, they are sweeping so that the stone will curl less and travel straighter. This type of sweeping is necessary if a team is trying to place a stone behind a guard, or if a team needs to hit a stone in a very precise location.
When the sweepers are sweeping for weight, they are sweeping to make the stone travel farther. Sometimes, the stone may be light (too slow), and will require sweeping to make sure it can travel the distance it needs. Other times, the weight may be excellent, but require a couple small sweeping adjustments so it stops in its desired location.
IMPORTANT: Sweepers cannot make a stone speed up, but can reduce the rate that the stone slows down.
Now that we have learned about a team sweeping their own curling stones, we will learn about where and when your opponent can sweep your stones out-of-play. The goal of defensive sweeping is to remove an opponent's stone from play, or at least move it farther away from the button. Defensive sweeping is performed like any other sweeping a curler would do on the ice. After one of your team's stones crosses the tee line, a player on the opposing team (often the skip) is allowed to sweep your team's stone. If the stone is moving at a good pace, defensive sweeping can cause the stone to end up out-of-play. Just as your opponent can try to sweep your stones out-of-play, you can try to sweep their stones out-of-play, only if the stone passes the tee line.
As soon as one of your stones begins to pass the tee line, your opponent is allowed to sweep. Whether a sliver of the stone or the entire stone is beyond the tee line, your opponent can sweep it until it stops moving, reaches the sideline, or fully passes the backline.
After your stone begins to pass the tee line, only one curler from the opposing team can sweep the stone. However, if there are multiple stones that can be swept defensively, an opponent may select one player per stone to sweep defensively. Thus, if two of your stones can be defensively swept, there can be two defensive sweepers on the opposing team, just as long as they are sweeping different stones. This rule extends to three and four stones that can be defensively swept.