So I guess that the inertia in the rotor gives you a chance to gain some altitude and at least accept a more reasonable landing spot rather than 12 o'clock below the nose.
Not so possible in a low inertia rotor like and R22 or H300 for example.
No, you are not using the inertia of the rotor system to gain altitude. The Nr stays at 100% the whole time (ideally).
You gain altitude by converting your forward speed into a climb.
You still wouldn't be able to gain a whole lot of altitude in a R22 because the whole thing is pretty light (haven't tried it), and you probably wouldn't gain very much in a 300 either because it's max speed is about 30 knots or so.
I did try it in a 44 once at the robinson factory. We entered at 110kts or so and easily gained over 400ft.