Dear Phllip,
Disabling a degree-of-freedom (DOF) in FAST (or the ElastoDyn module FAST v8) really means that the acceleration will be zero, velocity will be constant, and displacement will vary linearly (if the constant velocity is nonzero), regardless of the loads applied. Inside FAST, the equation of motion for the disabled DOFs are not formulated, resulting in no acceleration of those DOF.
Thus, disabling GenDOF will force the generator to rotate at a constant speed regardless of the torques applied. With GenDOF enabled, the generator will accelerate/decelerate if the applied torques are not balanced.
As discussed here, the RotTorq output of FAST (equivalent to LSShftTq) will equal the applied aerodynamic torque if the shaft acceleration is zero.
Best regards,