What does "auto-rotation" refer to in helicopter flight?

Auto-rotation in helicopter flight refers to a condition where the helicopter descends without engine power, allowing for a controlled descent. During auto-rotation, the rotor blades continue to turn due to the upward flow of air through them as the helicopter descends. This allows the pilot to manage the descent rate and land safely even when the engine is not providing power.

This process is critical in situations where the engine fails while in flight, as it enables the pilot to maintain control and execute a landing. The helicopter can generate lift through the rotors' aerodynamics due to the downward motion, effectively allowing the pilot to glide to a landing.

The other options suggest incorrect uses or techniques related to flight. The first choice implies a method of gaining altitude without engine power, which does not accurately describe auto-rotation. The third option references a maneuver to increase speed, unrelated to the concept of operating without engine power. Lastly, hovering techniques involve maintaining a vertical position in the air, which contrasts with the descent characteristic of auto-rotation.