I'd like to learn. Let me see if I have this right.
Energy (or work) is power times time: kilowatt-hours for electricity or horsepower-seconds in Robbie-speak, hp-s (not hp/s).
After engine(s) failure, the energy in the rotor will keep the aircraft flying for a second or so. Then it's gone and the rotor will stop, forever. It's how one adds energy to the rotor immediately that is the crux. In the cruise one can easily get some from the kinetic energy - motion - using the cyclic. In the hover one can't, one can get it only from descending - potential energy - and then only while the rotary wing is flying. Otherwise the aircraft is falling, not flying. Which introduces the other important factor - the angle of attack. Once the AoA has passed the stall angle the rotor stops, forever. So, lowering the collective lowers the AoA which helps keep the wing flying and slows the rate the energy is used by decreasing the lift and drag. Lowering the collective does not primarily add energy to the rotor.
Clearly entering autorotation in a genuine emergency is not as easy as falling off a log, or this thread wouldn't have started. More importantly, a helicopter wouldn't have punched through the solid roof of a pub, it would have landed. Somewhere else, probably.