I am not Marcus, but I'll have a go anyway as I am stuck in front of the computer.

1: Drag and force are not linearly proportional to each other over sails.
2: We must try to make the sails produce as much power for as little drag as possible, in the direction we want to go.

By opening the main you decrease the drag, and direct the force generated more in the direction you want to go, in my opinion.
More pressure is not always to the good if what it do is generate more drag and a force in the "wrong" direction.

Perhaps Marcus has some better way of explaining, or a better point of view <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />