What principle does the delta 3 hinge use?

Explanation from PART66Online

In a delta three hinge the flapping hinge axis is set at an angle to the perpendicular of the blade span rather than square to it, so that as the blade flaps the geometry automatically changes its pitch. That pitch to flap coupling damps the flapping and stabilises the rotor. The offset referred to here is the offset of the hinge AXIS from the perpendicular, not the radial offset of the hinge from the rotor centre, which is a separate design parameter that sets the hub moment. A triangular pitch lever and a flapping actuator are not the principle involved.