From the manual.
Autorotation is the term used to describe the flight condition which occurs when a helicopter's rotor blades ceasing turning under power and are instead driven by the force of the relative wind.
Like most helicopters, the Comanche has the ability to autorotate and continue flying for a short period of time even after its engines have been turned off or destroyed. When a helicopter autorotates, its rotors continue turning because of the force of its downward motion. The faster and longer a helicopter falls, the greater amount of air is forced upward through its rotor blades. Obviously, as more air is forced through the rotors the blades are moved with greater energy.
Autorotation won't get you back to base if your engines are destroyed but it can get you to the ground in one piece. It's tricky, though. You might want to practice autorotating a few times while you're still in training.
Climb to a point at least 500 feet AGL (Above Ground Level) and shut off your engines by pressing the ALT E keys. Your Comanche will begin falling but don't worry. As air starts rushing through your rotors they will begin turning. Eventually, the rotors will produce enough lift to slow your descent and give you some limited directional control. The tricky part of autorotation is judging just the right moment to "pull pitch" and land. Think of it as "flaring" a fixed wing aircraft right before touch-down.❞