From Whidden, 'The Art and Science of Sails',

"Addressing the big picture, a correctly trimmed headsail slows the wind in the slot just the proper amount, so the air on the lee side of the main does not separate when it is trimmed at a tight angle. A correctly trimmed main places the jib in a lift, meaning the boat can be pointed closer to the wind without the jib luffing. A correctly trimmed main also speeds up the leech velocities of the jib, making it a much more effective sail. This speeding up of the flow and the fact that the Kutta requirements are satisfied in a high speed flow region mean that flow around the jib is less likely to separate. Again, the theme of sail trim is to avoid separation."

Whidden does a good job of expanding Gentry's explanation of sail theory and interaction and the gist of it all is in the quote above. I recommend the book in general as a good read for those who may not already be familiar with it.

Chris