A turbine powered helicopter with a "draggy" rotor may not fully enter autorotation at low weight because the engine/s may not back off enough to allow the freewheel/s to disengage. The ECU flight idle setting may be quite high to ensure good engine response when going from lever fully down to a subsequent collective lever power demand, especially on older types.
The same aircraft at a higher AUM will have a higher rotor RPM in the same autorotative conditions (more "potential" energy being dissipated)and this will allow the freewheel to disengage. The aircraft will then fly in full autorotation, i.e. as has already been said, with the Nr/N2 needles split.