A downhaul is a line (rope) used to pull down a sail, spar, or sail tack to control sail shape and tension. It is crucial for adjusting the luff tension of a mainsail or jib, with increased tension flattening the sail for heavy wind and decreased tension adding power in light wind.

Key Aspects of a Downhaul:
Purpose: The primary role is to tighten the luff (leading edge) of a sail, which pulls the draft (curvature) forward and flattens the upper part of the sail to reduce power, or "depower".

Operation: In light winds, the downhaul is eased, allowing the sail to become fuller and more powerful. In stronger winds, it is tightened to reduce drag and prevent excessive heeling.

Components: While often acting on the tack of the sail, a specialized downhaul known as the Cunningham specifically manages luff tension on a mainsail.

Applications: It is used on mainsails, jib sails, and on sailboards (windsurfers) to create tension at the tack. On a spinnaker, a downhaul helps control the spinnaker pole.