Bethwaite's "High Performance Sailing" touches on this. Here is my take on it:

Sailing by the Lee allows you to present the leech (knife edge) edge of the sail to the wind, instead of the mast end. This avoids the normal "separation bubble" behind the mast, and can therefore generate 10-20% higher lift than mast-first trim. The problem is that the trailing edge needs a 0 to 7 degree angle of attack to not stall the airflow or jibe. Doing this optimally, with the correct trim top-to-bottom requires reverse-twist, which isn't practical, so it's only likely to pay off in conditions so light you can't get the the correct twist in the main by normal methods.
(It also requires light conditions so that puffs don't change the twist in the mast and stall the sail.)

Anyway, in the really light flat stuff, it can theoretically pay to adjust the *top* leech of your sail to have a 0 to 7 degree angle of attack to the apparent wind aloft. If you do this, you can theoretically get better performance than the mast-forward case.

But it's never paid off for me... since I don't sail in flat light conditions.