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Re: Flating reef
A flattening reef was used in conjunction with a racing shelf footed mainsail. The shelf foot had extra fabric sewn into the foot of the sail that made it more powerful in light air and off the wind. When sailing closehauled in strong winds, it made the sail too full, and the flattening reef enabled you to flatten the foot of the sail quickly and easily.
The flattening reef consists of only the cringle at the outboard end of the boom. Your crew is correct that the cunningham and the flattening reef are two different controls, not intended to be used together, but that doesn't mean that you can't use them together.
When you have both a flattening reef and a cunningham, you can use the flattening reef and cunningham separately for their normal purposes, or you can use them together as a short reef. I can't count the number of times I've been racing, and the boat was slightly overpowered, and I felt I needed to reduce mainsail area a little to keep her fast, but was afraid that the first full reef was too much. If you use the flattening reef and cunningham together as a short reef, you don't lose much sail area, but you reduce it just enough to settle the boat down, and you flatten the sail to the max.
The modern loose footed mainsail can be shaped with about the same fullness as the shelf footed mainsail, and I don't remember having seen a flattening reef on a loose footed mainsail.