Shape & trim
Cunningham
A downhaul at the luff that pulls draft forward and flattens the front of the sail.
A cunningham is a tackle that pulls down on a cringle just above the tack, adding tension to the lower luff without moving the whole sail down the mast. It is mainly a mainsail control, though some racing headsails use one too.
Its job is shape. As wind builds, the sail stretches and the deepest part of the draft wants to move aft. Pulling on the cunningham tightens the luff, pulls wrinkles forward, and helps bring the draft position back where it belongs.
It is not a magic flattening control by itself. The outhaul works on the foot, the backstay changes mast bend, and the cunningham works on the front third of the sail. Used gently, it is one of the cleanest ways to depower without changing sails.