You guys are killing me. How can I get any work done?
Found some more info on the subject
This image illustrates the change in wind speed as a function of height in different conditions. The 1st, nearly linear line is for wind speeds less than about 5kts.
Point is that optimal trim angle at the bottom of the sail would be different than the top - thus the need to allow twist and open the top portion of the leach.
Once the breeze goes turbulent, then you get the curves to the right in the above image, and no need for twist until you're overpowered.
Some good articles here:http://www.ockam.com/2013/06/02/wind-shear/http://www.skysailtraining.co.uk/sail_trim.htm
(the bottom section)