Think of a boat prop being dropped in the water by itself. I will spin as it falls through the water.
But the water is acting as the actual force spinning the blades..........a target or an animal will present friction and a resistive force.
I see a better analogy as a corkscrew vs a nail............much less force to push a nail through a cork then it takes to manually twist and turn the cork screw due to less area of resistence.
I am just having a tough time visualizing how it can possibly keep spinning once the head sinks in.