outhaul tension...
The outhaul is the other adjustment which has direct impact on rig performance.

Reduced outhaul makes the sail deeper in profile, generating greater power for light winds.

Greater outhaul tension flattens the sail profile, making the sail much easier to handle and faster in medium and strong winds.

Flatter trim allows the sail to pass more easily through the wind. The range between ideal strong and light wind trims may be up to 3cm.