The shaded area along the left axis signifies that you’re going too slowly for your height; should an engine failure occur then you have little to no chance of auto-rotating to the ground. Along the right of the bottom axis there’s another shaded area, this time signifying that you’re going too fast for your low height; if anything goes wrong you’ll smash into the ground before you get much chance to react.
Those shaded areas are generally called, in the UK, the avoid curve. I.e., you do what it says in the manual – you avoid flying in that state. In the US and Australia, they often call it something different: dead man’s curve. For the simple reason that if you’re flying in that state, say 200ft at 30kts, and your engine goes BANG, you are dead. Or will be after 200ft of vertical travel.
The downside? Sometimes you need to fly in that curve. Ariel cranes, for example, spend most of their working time low and slow, and for that reason have multiple engines. But for some jobs a big, multi engined craft wouldn’t work or would be too expensive, and there’s where the risk comes in. Like cattle ranching.
You know, I think that I’d really like to try it. For one day, and then say I’d done it and move on to a more sensible life…