Distant Blacks
Now you'll need to flip back and forth to see this step. Some of the dark rooftops were blowing that desirable sense of distance, so they're corrected here.
If you flicker back to the previous slide, you'll see the most distracting rooftops are now relieved, where "virtually black" had increased the clarity far too much.
Is "ten or more hours on a single work" starting to make more sense now?