A method of driving a main rotor of a rotorcraft in rotation while implementing an in-flight simulation mode that simulates failure of one of the engines of the rotorcraft. In simulation mode, and when a current speed of rotation (NR) of the main rotor is detected as being lower than a predetermined threshold speed of rotation (S), the simulation mode is kept active and a regulation command is generated in order to perform a controlled operation (A) of gradually increasing the power delivered by the engines by authorizing the limit imposed by a setpoint (OEI/2) for regulating operation of the engine in simulation mode to be exceeded. Said gradually increasing power is interrupted by the pilot staying under training and operating a collective pitch manoeuver of the blade of the main rotor providing a rotation of main rotor at the predetermined threshold speed in rotation.