It depends if the mast is of a prebend setup (spreaders rakes aft and diamond wires tensioned to bend the mast in the long axis) or "soft" rigged setup (spreaders almost perpendicular to the mast, low tension on the diamonds).

A prebend setup will resist bend in the short axis and accept bend in the long axis. By under rotating a prebend mast, you are allowing more bend in the long axis, thus helping to flatten the sail, depower. Over rotating reduces flex in the long axis and increases sail body/power at the cost of more drag.

With a soft rig, less rotation reduces the mast bend along the more flexible short axis, thus the sail is fuller/powered up. More rotation places the short axis such that the mast flexs more, thus flattening the sail more to depower it.

Mike.