Dear Peter,
RotTorq is the same as LSShftTq and is discussed at length here. Basically, rotor torque is the difference between the applied aerodynamic torque and the inertia due to rotor acceleration/deceleration. So, even though the aerodynamic torque may jump, the delta may be offset by the inertia due to rotor acceleration/deceleration. When the generator torque is held constant, the steady state RotTorq will equal this generator torque (multiplied by the gearbox ratio). The rotor acceleration/deceleration will be impacted by the rotor flexibility, which is why I suspect you are seeing differences between the NREL 5-MW model and your other model.
Until recently, the applied aerodynamic torque was not an output directly available from FAST. But we’ve recently added it as an output (named RtAeroMxh) in AeroDyn v15 within FAST v8.12.
Best regards,