By Bob Stern

When you tighten the mainsheet, you will increase the heel, and decrease the forward drive.
The weather helm, and the leeway will increase. The amount of "lift" felt by the jib is increased, and both the jib and main may stall. This will require that the boat be pointed higher, and this in turn further reduces the drive available. Over sheeting the main, is therefore to be avoided, with the exception of situations requiring that you really must pinch. Be forewarned, however, that you cannot keep this up for more than a few boat lengths, before speed suffers, and leeway increases to the point that you will loose more than you gain. In light airs, don't pinch at all.
Easing the Jib sheet. This will increase the jib's drive, while reducing its heeling force, much as easing- the mainsheet did for the main. In addition, it will increase the suction on the lee side of the main (its "power"), therefore increasing both its drive and heeling force. The change in heeling forces nearly cancel , and the result is a net increase in forward drive from both sails. The catch is that the main is "headed" less, leading to an increased tendency to stall. There will also be an increase in weather helm, due to the Centre of Effort moving aft. Generally, this is exactly what you want in light airs,. providing you can accomplish it without stalling the main. You may have to bear off to avoid luffing the jib.