crab,
It was me who asserted that it is there at all speeds. I did hundreds of maneuvers in 'Hawks and S-76 to understand the nature of maneuverability once upon a time, work that lead to an AGARD tech report and several AHS papers, and that helped lay out the Comanche's main and tail rotor systems. When next in the cockpit, (next week) I will repeat same to confirm.
My speculation then and now is that the engine/drivetrain system, including tail shafting, is all rotating the same way, and the delta torque of that system is a contributer, perhaps via a conservation of angular momentum. The effect is not in pitch in a hover, telling me it is not purely a main rotor effect (not H force, for example) and it follows roll acceleration and rate, even into rearward flight.
I will do some calculations based on inertias and velocities in a few days (when my ears stop ringing from having been boxed!)