Is this when a helicopter makes a normal controlled landing with the engine still functioning?When a helicopter loses power, the blades are still rotating and air is still being forced downward, but the blades slow down. Lift is reduced accordingly. The helicopter begins to accelerate downward. The air accelerated downward by the still-spinning-but-slowing blades meets the air still there from moments ago. There is viscosity. It has to go somewhere and that takes time and crucially creates a whack of drag as it works its way out from under the blades. If you are lucky, the drag created and ground effect add up to enough force to keep the downward velocity of your vehicle down to a survivable prang speed. If the blades stop spinning, almost all the lift force is lost and you prang in at something between zero and terminal velocity depending upon how high above the deck you were.
If the engine looses power, autorotation of the blades allows the pilot to land safely with the pilot adjusting blade pitch angle and forward air speed. Just as good as a parachute.
Descent with engine failure can be catastrophic if it occurs with no or little forward airspeed at a height particular for type of helicopter - generally speaking around 500 feet - for the air flow to be set up correctly from downwards movement to upwards movement over the blades.