Most rotorcraft use a fixed rotor speed (note that slowing the rotor speed could bring benefits in some flight situations, but that is not applicable here) flying different operating/flight conditions. The rotor is driven by the power turbine of the turboshaft through a (main) gearbox (reducing the turbine spool speed of several thousands of revolutions per minute to several hundreds). This fixed coupling makes that the power turbine and rotor speed are directly linked, so, maintaining a constant rotor speed implies a constant power turbine speed. Do note that the different flight phases of a helicopter require different power settings, e.g. the hoover requires more power (lift the whole mass of the rotorcraft) than a forward flight motion where the body generated lift. This mean that the power to the rotor is not constant, but depending on the flight condition. This power is generated by the power turbine and knowing that torque is related to the power by the speed:
$ Power = Torque \times Speed $
with a constant speed, the torque on the power turbine needs to be varied to get the appropriate power for the rotor.
A turboshaft engine consists of a gas generator and power turbine, these are coupled by the gas path, so, these can spin independently from each other.
To provide the gas power to the power turbine the gas generator creates a hot, pressurized stream of combustion products that drive the power turbine.
For that reason you see that the gas generator spool speed is low when the rotor requires less power (low fuel flow), or high (high fuel flow) if the rotorcraft is in a high power demanding flight condition like the hoover (there even is a difference in hoover in free air or just above the ground in terms of needed power!).