A "delta hinge" is a description of the way the blade is made to reduce angle of attack when it flaps. It is easy to do, just have the pitch change rod apply less pitch as the blade flaps up (increased thrust). The red blade below will flap about the diagonal line (where the hinge bolts are). That means as the blade lifts it has its feathering washed out, and the thrust is reduced.
The above is courtesy of Dave Jackson, and his excellent site (I hope he doesn't mind the linking, Dave, I'll pull it down in a heartbeat if you wish)
Delta hinges reduce the flapping forces on the blade and hub, so that the TR can be lighter and still withstand flight forces. The delta hinge works mostly in forward flight, were the advancing TR blade sees a bunch of lift increase, and the retreating TR blade sees the opposite. The hinge allows the disk to flap and even out the thrust across the TR disk, so it reduces stresses a bunch.
That flap to pitch coupling is called "Delta 3" by designers.