Targeted sweeping can keep the stone straighter or let it curl more. What research shows, what is open, plus rules, calls and common mistakes for sweepers.
The honest answer: curling is the only sport in which you can still influence the projectile's path after release, and you do it with the broom. A review of curling sports science puts it this way: sweeping changes the friction between stone and ice, so the stone can travel further, curl more or stay straight [1]. The author notes: sweeping can be very physically demanding, and effective sweeping needs a combination of downward force and brush-head speed, determined by the stone's velocity [1]. Also, sweeping to the left or right of a stone can keep it straight or let it curl more depending on its rotation [1].
How hard is it? The motion is easy to learn; timing and coordination with the skip are the work. There is no timeline. To be honest: the physics behind it isn't conclusively settled. A scoping review of 94 studies describes a controversy about the mechanisms of curl and sweeping [2]. So here you learn the handling and the rules, not a miracle formula.
First learn normal, hard sweeping. "Hurry" is the call that tells sweepers to sweep harder [5]. According to the review, the effect depends on downward force and head speed [1].
Milestone: You keep pace with the stone and sweep without pausing and without falling once across the sheet.
You may sweep in any direction; it need not cover the entire width of the stone. But the brush head must not be raised in front of a moving stone, you must not deposit debris in front of it, and you must finish to the side of the stone [5].
Sweeping to the left or right of the stone can keep it straight or let it curl more depending on rotation [1]. Which side goes with which effect is decided by the skip in combination with rotation and ice. Stick to what your team has agreed.
Behind the tee line (at the playing end) only one player from each team may sweep. This also applies to the opponent: for them only the skip or vice-skip [5]. Whoever arrives first has priority but must not obstruct the other [5].
The call comes from the skip, not from sweeper feel. Practise a stop sign and a sweep sign with your team. No call is given by radio: electronic communication is forbidden in the game [5].
Milestone: Your team hits the chosen spot with the same calls three times in a row (practice).
Don't sweep against the skip, but with the rotation in your head. A stone with clockwise rotation curls to the right [3], and sweeping on one side or the other changes how strongly [1]. Before the game, fix what your skip means by "straighter" and what by "more curl". That saves the discussion when it matters.
Sweeping changes the friction between stone and ice. The stone can then travel further, curl more or stay straight [1]. Which side you sweep depends on the rotation [1]. The exact physical mechanisms are still disputed in research [2].
With downward force and fast brush-head movement; both depend on stone speed [1]. Rules: any direction, don't raise the brush head in front of the stone, don't deposit debris in front of it, finish to the side [5]. Practise this with a coach.
One player per team at a time. For the delivering team it can be anyone, for the other team only the skip or vice-skip [5]. You may sweep opposing stones only once they have reached the tee line at the playing end, and only behind it [5].
Between the tee line at the delivery end and the hog line at the playing end: if the team or its equipment touches the moving stone, it is removed immediately [5]. If the opponent touches it instead, it is redelivered or placed where it would have come to rest [5].
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