Not qualified to speak as an engineer but will take a stab at it.
Imagine a mast with straight spreaders and no diamond wire tension. It is straight.
Start tightening the diamonds. Which way does it bend? Probably pretty random depending on which side is tighter and/or slight bends in the 'straight' spreaders.
So now sweep the spreaders back. The tension from the diamond wires are now definitely pulling the mast in a particular direction. So now when you tighten the downhaul (shortening the distance between the foot and head of the sail) the leverage you are applying is already oriented in this particular direction.
Now to complicate this a bit more. Longer spreaders will give you the same bend with less diamond tension. Less diamond tension will result in a less side to side support (the original purpose of the diamonds). This is when rotation comes into play.
Or so I think.
Clear or mud?