In case anyone else is interested in the not-at-all-obvious-answer [1], I’ll save you the google: autorotation
> Autorotation is the ability of a helicopter to in some situations have its rotors powered by the flow of air rather than the engine. In short, as the helicopter descends, the upward directed flow of air pushes through the rotors, makes them rotate faster, which in turn provides lift.
a helicopter can "glide down" just like an airplane. if your engine fails, with the aid of the rotor as you descend you can turn the potential energy of being up in the air into forward motion, and that forward motion will generate lift over your rotor blades. You practice these techniques as part of getting a license.
> Autorotation is the ability of a helicopter to in some situations have its rotors powered by the flow of air rather than the engine. In short, as the helicopter descends, the upward directed flow of air pushes through the rotors, makes them rotate faster, which in turn provides lift.
[1] https://www.highskyflying.com/can-helicopters-glide/