But it still does not make sense to me how when a blade has positive flapping that increasing the pitch will reduce that flapping the same way as decreasing the pitch as with positive delta3. And of course while they explain very basically why it works they don't explain why they chose it over a positive delta3 tail rotor or why they chose 30 deg for the neg but 45 deg for the positive ones.
One explanation could be the in plane oscillatory loads: A positive delta3 will increase inplane oscillations, when a negative delta 3 will decrease in plane oscillations when the blades are flapping. So , for the drive shaft, couplings, t/r gear boxes, a negative delta 3, means less torsional stress and better component life. The choice is probably only driven by mechanical reasons