The purpose of the delta 3 hinge is to modify the flapping frequency of the rotor away from its natural frequency. A two-bladed T/R has a natural frequency of 1:1, and the aerodynamic loads are 1:1 causing a high flapping amplitude. A positive delta angle (reducing blade angle on up-flap)increases flapping frequency above 1:1. negative delta angle (increase blade angle on up flap)reduces flapping frequency below 1:1, Both accomplish the same goal, both are used on tailrotors, Bell 212 has negative delta, Bell 206 uses positive. Unfortunatley I'm not smart enough to tell you why you would use one over the other.