People who are new to curling, or unfamiliar with its nuances, are often puzzled at the fact that players wield brushes and sometimes sweep the ice furiously as the granite stone is delivered down the ice. It may look slightly odd but the sweeping is an integral part of the sport of curling. Indeed, I remember a very good curling player once telling me that a good stone in curling is one that requires to be swept. The rationale is that you have some control over where the stone will ultimately end up, so can increase your chances of getting the stone (or rock) to come to a stop exactly where you wanted it.
To understand why sweeping provides a degree of control, it's important to understand the role that friction plays in curling. There is friction between the ice and the underneath of the stone which means that a slightly underpowered shot will come up short. By vigorously sweeping the ice, the curlers can momentarily melt the ice. This lessens the amount of friction and this has two effects. Firstly, it reduces the speed of deceleration, meaning that the stone will end up going further than it otherwise would have. Secondly, it will cause the stone to stay to a straighter line. The stones curl most when delivered at a slow pace, so by maximising the stone's speed, the players can reduce the amount that the stone swings from one side to the other.
One example of where sweeping is often employed is on a take-out shot. The aim is to knock your opponent's rock off the ice. However, if the player's shot is in danger of curling to much, the sweepers will sweep furiously in an attempt to straighten up the shot and ensure that the takeout succeeds.
So, having covered why players sweep, let's move on to how to sweep. It's pretty straightforward actually. The curling brush is a long-handled brush, which is held two handed. You will have one hand lower down on the handle than the other and will then sweep the brush horizontally just in front of the stone as it makes its way down the ice. It's important that your brush head doesn't touch the stone and that your feet don't either, as any contact with the stone will render the shot null and void. You also want to stay pretty close to the front of the stone to maximise the effect of the sweeping. Sweeping too far in front of the stone will be ineffective.
Usually, two players will undertake the sweeping, although occasionally the player whose shot it was will also join in. For the two assigned sweepers the usual approach is to have one on each side of the ice. They will come in and take their positions, deciding which of them is going to place his brush closest to the stone.
The technique of sweeping can be enhanced by having the lower of your hands quite far down on the brush handle and bending over towards the ice. This will mean that all of your weight will be transferred down through the broom and result in a more powerful sweeping motion. The other key technical element is your movement down the ice. For slow shots, it's easy enough just to walk alongside the stone and some amateur players do this. However, you will sometimes have occasion to sweep shots that are travelling faster and to stay level with the stone, you will need to slide smoothly along the ice. You will be standing sideways to the stone with your feet a little apart. To slide you will allow your lead foot to slide sideways and then slide your other foot after it. To begin with, the chances are that this will take a bit of perfecting and you may have to leave the faster shots to the experts. Be careful not to over-exert yourself as I've seen quite a lot of falls happen when players have valiantly attempted to keep pace with a stone that it travelling faster than their comfort zone.
So that's a quite summary of the importance of sweeping in curling and how it is achieved. It's worth watching some international curling to see the speed and exertion that goes into sweeping at that level. Even at amateur level, the sweeping component leads to an excellent workout. You will certainly know all about it if your team-mates play two or three stones in a row that require end to end sweeping! It's also extremely satisfying and exhilarating when you succeed in coaxing a stone into the desired area, knowing that without your efforts it would have been a poor stone.