Sweeping makes curling a team activity, and allows all four players on the team to work together with the goal of reaching a better outcome for each shot than can be achieved without sweeping
You sweep for two reasons:
The main problem people often have with sweeping is that one can easily over-sweep a stone. Good curlers need a keen sense of observation to know when to sweep, and when not to sweep. Some curlers even use stop watches to inform them of the speed of a rock.
Additionally, a delivered stone that is too heavy makes sweeping useless. It is traditionally best to deliver your rock with 90% to 100% of the distance/weight needed on a draw shot, allowing your teammates to sweep the stone (if needed) to the precise desired distance.
To see what Olympic Curlers Debbie McCormick and John Schuster have to say about sweeping click here.
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