Its my understanding that Head Bolt is correct.
Flap Forward, as aerodynamic force, does not actually exist.
The disc actually pitches (flaps) forward because of a decrease in the value of Flap Back (which is an aerodynamic force).
As HB describes your cyclic is positioned (forward) to counter for a certain value of Flap Back. As you lower the collective you reduce the value of Flap Back....the cyclic is still positioned forward to maintain equilibrium (constant airspeed) and it appears has if the disc is flapping forward.
Its not flapping though, the forward cyclic position is causing it to pitch (flap) forward.
I do admit that it is somewhat semantic......as obviously there is an aerodynamic force at play whether its increasing or decreasing.