Originally Posted by ScottUK
So from what I gather the mainsail is flattened at the leading edge in the area of the mast bend and then the leech would not have as much tension thus adding more twist and spilling more wind. It has been what I thought but have heard conflicting information.
I understand moving the top of the forestay aft and down would also decrease the tension on the leech of the headsail and so induce twist. Not sure about flattening the headsail though. If the foreatay tension is tight and is moved aft through increased backstay tension wouldn't this just decrease the angle of the straight line of the forestay and not reall effect the luff. But I guess it would all depend on the initial forestay tension.
As a broad general rule, as the wind increases the headstay sags more, and this is especially true on a fractional rig where the increased headstay forces induce more mast bend. When you tighten the backstay, this increases the tension on the forestay and that reduces this sag. When you reduce sag, almost by definition, the stay itself moves forward and to windward. And this increases the distance between the leech of the sail and the stay. As that distance increases, the jib is flattened and therefore depowered.