i dont agree with Will R...
while this is all related... its not all the same result or goal.
prebend is used to match your luff curve of your main... set for your sail.
downhaul is used as an on the fly adjustment to power or depower.
Nearly, there is more to it than that.....
Yes, prebend is used to match the mast bend to the luff curve to get max power, but you can also loose power by adding MORE prebend.
The diamond tension only (reallY) induces bend between the top and the bottom of the wires.
The downhaul streaches the sail (A little) ans then bends the mast along the WHOLE mast as the sail is attached at the top.
As others have said, prebend setting (and spreader rake) is the course tune for the mainsail as it is difficult to change (except on some A classes and F16's that run adjustable diamonds from the wire). You then use the outhaull, downhaul, mast rotation, mainsheet tension and traveller position to control the shape and power of the mainsail.
