Autorotations are simulated emergency or crash landings. And it requires that the helicopter engine power is cut off from the rotor blades (you will still hear the engine because it is kept on).
Auto rotations are practiced a lot because pilots need to know how to land helicopters safely when there is no engine power continuing the movement of the rotor blades. As you can imagine, these hunks of metal are not aerodynamic like airplanes. In fact, its really hard to imagine that they can fly at all. The extremely powerful engines together with masterfully designed rotor blades make it possible. So, you really have to work with them even in a failure situation in order to have a chance to land safely.
You have to catch the failure at the right moment (before the blades come to a full stop) so that you can use the remaining momentum and a pin wheel effect to get the ship safely to the ground.
This is not for the faint of heart!
These five landings are being done in a Robinson helicopter at Lawrence Municipal Airport in Lawrence, MA.