Dave / Tom: Dave : your almost right but your missing one piece of the equation, which I'll explain in my explanation to Tom. I've never read Tom Whidden but if he says that INCREASING tension on the back stay INCREASES the belly then I would have to disagree with him and so does Dennis Conners, Gary Jobson, Bill Gladstone, Ivar Dedekam and everyone else I've ever read. Tom: I can understand your confusion because I spent the first 4 of the last 8 years mystified about some of this sail trim stuff. For example, it was hard for me to understand that LESS twist equals MORE power and MORE twist equals LESS power. Mast Bend also confused me but a lot of this stuff made sense after I tried the concepts on my boat. Here's the piece your missing on mast bend and hopefully my explanation will clear up your confusion. A mainsail is called triangular but it isn't. The sail is actually constructed with an exaggerated leech (roach) and a curved luff (luff curve) and the luff curve can vary considerably. Masts are bent because of the luff curve so when you bend the mast to try to match the luff curve, the mainsail gets flatter, especially at the leading edge and in the middle to upper parts of the sail. When sailing upwind, you bend the mast to make the sail flatter and when sailing off the wind you straighten the mast and the sail goes to its NATURAL FULLNESS. So, again, more mast bend flattens the main by taking out the natural fullness and less mast bend makes the mainsail fuller by putting the fullness back in. A straight mast allows the mainsail fabric to sag into the distance between the luff and the leech thus giving the main a deeper shape. Bending the mast flattens the sail by drawing the fabric across a greater distance. For a better explanation, the next time you are in a book store, take a look at Dennis Conners book "Sail Like A Champion". See diagram #4.17 on page 118 and the explanations on pages 124 and 146 and then try the adjustment on your boat and see what happens. The function of the boom vang is to control TWIST and DRAFT POSITION. One other point, and that is when you bend the mast to adjust the mainsail you are changing the setting for the jib and that requires a not set of corrective measures.