The idea behind this must be that each foil section has it's own unique lift versus drag cutve. For instance a NACA 008 foil has a specific angle of attack where performance is optimum (highest lift to drag ratio). Due to practical design and arragnment considerations boat foils cannot usually be positioned to operate at this optimum angle. The gybing dagger boards are a way to improve the situation and bring the fiols closer to the best angle of attack.
When looking at these angles, the leeway (yaw) angle of the boat (angle of the boat with respect to true path through the water) needs to be added to the the angle introduced by the gybing action of the boards to come up with the angle of attack seen by the foil. In other words - need to take into account that boats don't sail in a straignt line.