I assume you have a sliding gooseneck. If so, then the gooseneck should just hang down as far as the luff (leading edge) of the sail allows. Below the gooseneck there is usually a small SS ring welded to the gooseneck where a small line can connect, which will go down to a cleat installed on the mast somewhere between the mast base and the sail entry slot. This line is sort of a cunningham too (more later). So, after you raise the mast, you stretch out the leading edge of the sail with this line below the gooseneck to suit the wind conditions. That way your gooseneck stays in place while sailing. When you lower your sail at the end of the day, the gooseneck will fall down to the location of that cleat, which usually is installed in the slot of the mast. If you don't have one, I would add one. A true cunningham is a hole a short way above a fixed gooseneck in the sail where a line can pass through, or a SS hook, which usually also has some mechanical advantage with the use of some blocks. When pulling on the cunningham line, you tighten the leading edge of the sail, much like pulling harder on the halyard. The cunningham is used more frequently by racers and often pulling or releasing a cunningham is quicker than messing with the halyard, which might have to be put on a winch to adjust it. Well, with a sliding gooseneck, this line that goes to the cleat below the gooseneck is a way to adjust the leading edge of your sail by either loosening it or tightening it. The same effect can be done by adjust your halyard and then you can leave the line attached. When you adjust the leading edge of any fore and aft sail (like a genoa, jib or mainsail), you adjust the draft of the sail fore and aft. Tighten the sail luff and that will bring the draft forward, which is good for power and usually as the wind gets stronger, you tighten the luff. If you get a curl in the luff, then that is from too much tension and so then you must release the halyard so that goes away. For light air, you usually want the draft to go aft, so you loosen the halyard. The same is for the outhaul on your main. If your outhaul is adjustable, then you can easily tighten or lower the foot of your mainsail. By tighening it, you will then pull the draft down and by loosening it, you will ease the draft up. Again, in heavy air you usually want to tighten the outhaul and in lighter air you loosen it. Hope this helps you trim your main properly by thinking of the right stuff and not the wrong stuff.
