When flying an aircraft, you must always take into account the possibility of a sudden loss of power/thrust.
The engine failure is a key point in flight safety and a pilot must always be trained such as recover with the available techniques from such a situation.
During an autorotation, the engine is no longer supplying power to the main rotor but the pilot can still collect some lift from the rotor.
This is a main engine failure recovery technique for an helicopter before anticipating a forced landing.
In other words, it occurs when the pilot pushes the collective pitch trim totally and the helicopter passes from state 1 with power to state 2 without power.
In autorotation, the reversal of the induced velocity adds power to balance the profile power losses in the rotor, as indicated below:
where T(vi) is the induced power and P0 is the profile power. Since the induced velocity is reversed, vi would have a negative sign and the total power is zero. So it confirms the definition.
During vertical autorotation, the rotor disk is divided into three regions:
The following scheme shows three blade sections that illustrate force vectors in the driven region A, a region of equilibrium B and the driving region C: