Holding the line means keeping body and stone on the track to the broom until release: hack alignment, hips, head, release and concrete checks with milestones.
The honest answer: holding the line means body and stone stay on the track to the broom, from the hack to the release. Curling Canada defines the line as the stone's initial direction: it starts on a straight path and only begins to curl after the break point [1]. If the stone already runs crooked on release, you can no longer factor in the curl.
According to the national coaches, the goal of the delivery is to hit line, weight and rotation consistently [1]. Holding the line is only about the first part. A club glossary simply defines "line" as the delivery target [2].
How hard is it? You need a calm set-up and a stable slide. The finer points are then habit. There is no timeline. The lever almost always sits in the body: if you are cleanly aligned in the hack and keep your head still, you hardly need to steer the stone later. And the national coaches also say: there is no single uniform technique, as long as the stone ends up with the line, weight and rotation you wanted [1].
Set the toe in the hack so it points to the broom, make sure the hack knee also points to the broom and the shoulders are square to it [3]. When aligning, the body moves, not the stone: for targets at the outside of the house the stone shifts at most about 2 inches (5 cm) [1].
Milestone: From behind, your partner sees a line: toe, stone, broom.
With the stone in the middle of the hack, stone and body share the same line of push (hack knee behind it), so the lines hardly need to converge [1]. If the stone sits further toward the centre line, stone and body lines must converge later [1]. Pick one position and stick to it.
Plant the sliding foot exactly on the target line, with the inside of the ankle on the middle of the stone [1]. Hips and shoulders stay square to the target line [1]. The head sits centred on the target line [1].
During the slide the stone moves straight along the target line [1]. Just before release, the curler directs the stone from the middle to the outside of the target broom head until separation [1].
For advanced curlers, Curling Canada recommends imagining a lane instead of a single line (two lines, about one broom head wider than the stone). That leaves room for pure, mini-set or set releases [1].
Film yourself from behind, the camera centred on the target line. A still frame at release shows you whether head, shoulders and sliding foot lie on one axis.
Milestone: Of ten stones with the same broom, at least seven visibly lie in the same track on the way to the broom (practice measure, not a standard).
Speed can make the line easier. In 20 elite curlers, lateral sway in takeouts was lower than in draws, and the higher the push-off speed, the more stable the sideways position (r = -0.46) [6]. This is no instruction for beginners, but it explains why a timid, too-slow draw often looks wobblier than a decisive shot.
The line is the stone's initial direction: it starts straight and only begins to curl after the break point [1]. In a club glossary, the line is simply the delivery target [2].
Typical signs are: you slide down the centre line although the broom is to the side, or you drift toward the sideline [3]. Align the toe in the hack on the broom and move the stone in a straight line to it [3].
Both are allowed. The middle has the advantage that stone and body share the same line of push and hardly need to converge [1]. Each side has advantages and drawbacks; the curler or team must decide [1].
Little: for targets at the outside of the house the stone shifts at most about 2 inches (5 cm); mostly the body moves [1]. Aim at the broom and take the body with you.
Sweeping for Line in Curling: Influencing the Stone's Direction