There are two interconnected mechanisms to power and depower a sail using mast rotation alone:
1) Changing the section efficiency of the sail/mast airofoil by means of rotating the mast with regards to its most efficient angle for the conditions. Both under rotating and over rotating will reduce the efficiency of the airfoil, but over rotating costs more drag.
2) Changing the mast curvature using the different mast section flexibility in each axis. The mast's beaviour may be tunned to a certain extent using more or less pre-bend, as posted before.
Usually you want both mechanisms to work in synchrony, so that when you under rotate the mast, the airfoil loses power for both reasons simultaneously: due to the mast being more aligned to the sail and to the sail becoming flatter with the greater mast curvature.
There is one thing people neglect: it is necessary to know how much rotation is ideal for your mast in order to identify when it is under or over rotated. The ideal rotation is not a fixed figure. The ideal rotation of a 5% chord mast is approximately 53 deg, falling fast to 35 deg when the chord is 10%. Although possible, this is difficult to evaluate by just looking at the mast and sail. You need to divide the mast chord by the sail foot to find the chord % and figure the ideal angle. (take a look at the attached drawing/table copied from Tom Speer's text on the subject). Only after this is known and marked, can the pre-bend be tuned accordingly to make both mechanisms work in synchrony.
