Shape & trim
Outhaul
The control that pulls the clew aft along the boom to flatten the foot.
The outhaul pulls the clew aft along the boom. On a mainsail it is the main control for the lower third of the sail: ease it and the foot gets rounder, pull it on and the lower sail gets flatter.
That change is easy to see on a loose-footed main, because the cloth can lift away from the boom. On a foot-bolted sail the adjustment is still real, but the groove and bolt rope share the work and the change looks less dramatic.
In light air, a little depth low down helps the boat accelerate. As the breeze rises, a tight outhaul reduces draft, heel and weather helm. It does not replace halyard or cunningham tension, because those act on the luff. It is a foot control, not a whole-sail cure.