The outhaul is a line from the mainsail clew to the end of the boom. It adjusts the tension of the mainsail's foot along the boom, primarily controlling the position and depth of the draft in the lower half of the sail. Tightening it moves the draft forward and flattens the sail to reduce drag and heeling in high wind or when beating upwind. Easing it moves the draft aft, adds depth or fullness to the draft, and powers up the sail when reaching, in light air, or in waves.
Most useful on a loose-footed mainsail, the outhaul might not be as effective if the foot of the sail has a boltrope. In light air or choppy conditions, ease the outhaul to give a fuller, more powerful sail shape. As the wind rises, harden up the outhaul to flatten the sail.