How does a Delta-3 hinge on a helicopter tail rotor work?
It's because the blade's span-wise axis is "swept" relative to its flapping hinge line, so when it flaps, the sweep angle results in the blade's effective AOA changing somewhat (for the advancing blade, reducing; that's what the feathering part means).  

To picture it, imaging you are standing directly in front of the tail rotor disc watching the advancing blade of the plain rotor come over the top or bottom straight toward you.  All you see is its thin leading edge.  If the plain blade flaps left or right, still all you see is the thin leading edge because the blade's axis is perpendicular to the feathering hinge axis.

Now imagine the Delta three rotor blade from the same vantage point, and the hinge flaps left or right. Because the blade's span-wise axis is offset, or  "trailing" the flapping hinge axis, you will be able to see some of the upper or lower surface of the blade from your vantage point when it flaps. To the airstream, this is effectively a slight reduction in AOA ("feathering") compared to the AOA of the unflapped blade as set by the pitch change linkage (or an increase when flapping while retreating).