When to keep thinking

The diagrammed position is from a book I recently acquired. It was used to demonstrate a point about calculation and thinking, and is White to play and ...
Of course if I told you the desired result that would make the problem far easier, but even now I've already given you a big clue. But this kind of illustrates the point I'm trying to make, in that we often don't know when to keep thinking.
If we know the problem to solve, we will try and find a solution. If we don't, we are often happier with a lesser one. And sometimes, just thinking about a problem, without knowing our goal, helps us recognise that a better solution may be obtainable.
It turns out the best line in this position isn't very long, but it might be hard to spot the key idea. Of course if the position just before the key move was required was presented it would be a far easier problem. So the solution does not so much hinge on the key move, but instead knowing one is required!