Try not to get all caught up in theory. Keep it simple. Think of a mast with exaggerated prebend. The less rotation you have the flatter the sail since the mast is bowed out so much. As you rotate, the tension on the sail eases allowing for a fuller sail. You don't need to worry about all that other stuff when you're just getting started.......

Basically, if you have the prebend correct, less rotation to depower like in heavier air. (flattens the sail) More rotation for a fuller sail (more power) as the wind speed decreases.

Test this on the beach next time you sail. Do some dry sailing. Turn the boat into the wind like it would be while sailing up wind. Sheet in, set downhaul, then set for lots of rotation . Then get behind the boat a ways so you can see the effect on the main. Now setup for minimum rotation. then check sail from behind the boat again or from which ever angle helps.

Easing the sheet will also give you a fuller sail.

You can also think of an airplane on takeoff with flaps down for a " fuller" wing.(more power/lift) As the speed of the plane increases the pilot " flattens" the wing (brings in the flaps)

Don't forget to adjust the downhaul as well.