[midwesterner]

It bends the mast, which pulls the middle of the mast forward and the head of the mast aft and downward. It tensions the forestay as well. When the mast bends, the middle of the mast moving forward pulls fabric out of the sail horizontally, thereby flattening the sail as needed in higher winds. The head of the mast moving aft and downward loosens the leech of the sail changing the angle of attack and so reduces leeway, weather helm, and heel.

I would like to better understand the use of an adjustable backstay. My boat, a Bristol 35.5, has a hydraulic backstay adjuster. It is easy to control while steering. There is a pump handle to tighten it, and a small knob to release. I am confused about the information stated above.

You said, "the middle of the mast moving forward pulls fabric out of the sail horizontally, thereby flattening the sail as needed in higher winds".

It would seem to me, that as you put more tension on the backstay, and the mast is pulled aft, the sail would slack and become less flat. I would like to better understand the use of my hydraulic adjustable backstay.