An often overlooked component of a headsail is the sheet lead position. While some people treat these blocks as static and permanently set attachment points, the truth is, they are meant to be moved around depending on what you are trying to achieve with your headsail.
Sheet leads are typically only able to move fore-aft, but some yachts are equipped to allow arthwartship adjustment as well. Fore-aft is by far the most common form of adjustment, where the sheet block is setup on a car mounted to a track.
In the simplest of ways, moving the car forward is ideal in light air situation while moving the car aft is ideal in heavy air situations. The reason is, as you move the car forward, the force of the sheet is transferred up into the leech of the sail, allowing the foot to fill in. This creates a bigger chord in the sail and generates more power. When you move the sheet block aft, the force of the sheet gets transferred over to the foot of the sail. This will cause the foot to flatten and make the sail flatter. A flatter sail will generate less power and thus allow you to maintain control as the winds build.
As you transfer the force of the sheet from foot to leech or leech to foot, the other side of the sail becomes ignored by the force. So when the car is forward, the leech is under control while the foot is ignored. When the car is aft, the foot is under control and the leech is ignored. This can serve some additional benefits as well.
When winds build, you want to flatten the sail by tensioning the foot. Moving the car aft will accomplish this and it will also alleviate force on the leech. This will cause the sail to twist and the opening leech will spill excess air from the top of the sail. In high winds, this is ideal and will allow you to continue sailing along comfortably and safely.
When you are reaching and have eased the sail, you may find that your sail is developing a twist, even though you don't want it to do so. Moving the car forward will allow you greater control of the leech and grant you the ability to close the twist in the sail, maximizing the power from the headsail.
As of now, we understand that moving the car forward will control the leech and moving aft will control the foot, but where is the car to go to control both? The answer is somewhere in the middle.
As you move the car forward and aft, the angle the sheet makes to the sail will change. With the car aft, the sheet will come into the sail at a very shallow angle. If you continue this line, it will meet the luff of the sail somewhere down near the tack. As you move the car forward, the sheet will meet the sail at a more extreme angle and the imaginary line will reach the luff further up towards the head of the sail.
The neutral position where the force on the leech and foot are about equal is when the imaginary line that extends forward of the sheet meets the luff at about 40% the height of the luff (from tack to head). This point is considered to be the neutral position for your sheet block, and anything forward of this position is for lighter airs while anything aft is for heavy airs.