In light air or in rough seas, the sails should be full. When beating in a stiff breeze, however, and unless there are waves, the sails should be flat. An adjustable backstay is used to bend the mast and thereby flatten the mainsail as the center of the mast moves forward while simultaneously the mainsail leech gets looser, opens up, and hooks less to windward. This works only on a fractional rig, of course, where the forestay attaches below the masthead.
When the mast is straight, the mainsail leech is tight and it deflects air on the windward side of the main, bending the flow back to windward. This pushes the stern to leeward, causing weather helm. Applying some backstay tension matches the curve of the mast to the curve of the mainsail, thereby reducing the draft. It opens up the leech and directs the airflow straight astern or even a bit to leeward, reducing weather helm.
If the headsail has roller furling, tensioning the backstay will make the headsail easier to furl and will keep it neater and more compact when furled.
When applying backstay to flatten the mainsail, it is often useful also to flatten the jib by tensioning its luff or moving the jib sheet fairleads aft.
backstays
Running backstays, also called baby stays or runners, are adjustable lines that stabilize the middle of the mast and keep it from bending forward, especially from the pull of a staysail forestay. Without this aft tension the mast might bend too far forward, pump in a seaway, or even fail. Their support also allows higher tension in the jib forestay for better sail shape and reduced drag when beating upwind.
Common on fractional rigs and boats designed for offshore sailing, running backstays require constant attention. Crew must check—or set—them after every tack or gybe, hardening the windward one while easing the leeward one so it does not obstruct the boom. They typically consist of a high-load wire or synthetic line (eg, Dyneema) attached to the mast, connected to a tensioning system (block and tackle or winch) in or near the cockpit.