I'm not sure about the '60 knot' reference, but it does make sense that the aircraft must
have speed to encounter a low G mast bumping scenario. At cruising speeds, when the cyclic is pushed forward abruptly, the nose pitches down but the aircraft's momentum allows it to continue on its previous trajectory. This reorients the oncoming flow
of air more perpendicular to the rotor disk, reducing individual rotor blade angles of attack, and ultimately (if the pushover was aggressive enough) a loss of rotor thrust. Now at low speeds or in a hover, there is not enough momentum involved, and the helicopter simply begins a decent as the cyclic is pushed forward (the disk remains
sufficiently loaded). That's the way I see it anyhow. As for the comment about LTE above which stated 'the tail rotor is stalled...', I have to disagree with that as well. Pick up the AC on LTE and study the 3 critical wind azimuths which produce LTE. In no instance has the tail rotor stalled, but rather, the demand for tail rotor thrust has exceeded that which is currently available.