To "back a sail" is a nautical maneuver where a sail is positioned to catch wind on its forward (leeward) side, pushing it against the wind rather than filling it normally. This creates backwind, causing the bow to turn away from the wind, used specifically for tacking, turning in tight spaces, or "heaving-to".

Key Details on Backing a Sail:
Purpose: The technique is used to push the bow around quickly during a tack, or to stop a boat's forward momentum and hold position (heaving-to).

Action: The sail is held or sheeted over to the windward side, contrary to its usual operating position.

Effect: The wind strikes the side of the sail that usually faces away from the wind, forcing the boat sideways and causing it to pivot.

Alternative Definition: Note that a "backing wind" refers to a separate meteorological phenomenon where wind shifts counter-clockwise.

This technique, sometimes used in "back and fill" maneuvers to navigate narrow channels, is an essential skill for enhancing maneuverability, particularly in older or large sailing vessels.