Its when the top 3rd or more of your sail inverts . Once I started sheeting (regardless of DH), my sail would invert and never really flatten out--very slow!.--This happened regardless of whether I used stiff or flexible battens. Taking out some of the Pre-bend--seemed to make much of the problem go away--except when ALOT of DH and Sheeting is applied



I seem to have more of a range new between a full sail and a flat sail when needed.