Method of driving in rotation at least one rotorcraft rotor (1,2) by a power plant with which the rotorcraft is equipped, said method comprising the following operations: -) identifying by a control unit a required speed (Nr) for driving in rotation at least one vertical-axis main rotor of the rotorcraft, the required speed (Nr) being calculated by applying a first calculation rule incorporating at least one first calculation parameter giving rise to a controlled variation, in a predefined range of values, of the value of the required speed (Nr) depending on a variation in the value of the first calculation parameter, -) applying by the control unit a method of determining by calculation a power setpoint to be delivered by the power plant, the calculation of said power setpoint being based at least on applying a second calculation rule incorporating at least one second calculation parameter for calculating anticipated power (Pf) that the power plant must deliver as a function at least of loads supported by the main rotor driven at the previously-identified required speed (Nr), -) transmitting at least one control order relating at least to said power setpoint to regulating means for regulating the operation of the power plant, said regulating means causing the power plant to be operated to drive said at least one rotor (1,2) in rotation in accordance with a driving of the main rotor in rotation at the required speed (Nr), said regulating means comprising at least one regulating unit for regulating an injection of fuel to at least one combustion engine of the power plant, characterised in that the second calculation rule for calculating said power setpoint (Pt) incorporates at least one third calculation parameter for calculating a power surplus, said power surplus relating to a progressive power need to be delivered from a current power (Pc) for driving the main rotor at a current speed of rotation to the anticipated power (Pf) for driving the main rotor at the required speed ( Nr). -) identifying by a control unit a required speed (Nr) for driving in rotation at least one vertical-axis main rotor of the rotorcraft, the required speed (Nr) being calculated by applying a first calculation rule incorporating at least one first calculation parameter giving rise to a controlled variation, in a predefined range of values, of the value of the required speed (Nr) depending on a variation in the value of the first calculation parameter, -) applying by the control unit a method of determining by calculation a power setpoint to be delivered by the power plant, the calculation of said power setpoint being based at least on applying a second calculation rule incorporating at least one second calculation parameter for calculating anticipated power (Pf) that the power plant must deliver as a function at least of loads supported by the main rotor driven at the previously-identified required speed (Nr), -) transmitting at least one control order relating at least to said power setpoint to regulating means for regulating the operation of the power plant, said regulating means causing the power plant to be operated to drive said at least one rotor (1,2) in rotation in accordance with a driving of the main rotor in rotation at the required speed (Nr), said regulating means comprising at least one regulating unit for regulating an injection of fuel to at least one combustion engine of the power plant, characterised in that the second calculation rule for calculating said power setpoint (Pt) incorporates at least one third calculation parameter for calculating a power surplus, said power surplus relating to a progressive power need to be delivered from a current power (Pc) for driving the main rotor at a current speed of rotation to the anticipated power (Pf) for driving the main rotor at the required speed ( Nr).