Imagine that you have to solve
an exercise which consists
of a Mate in one move.
You have to find only one move.
And this move must be unstoppable.
Therefore only few pieces
can be implicated.
And no move of the opponent
can be an obstacle.
This double condition erases
many possibilities
and this means
that even a brute force method is possible.
But this is unavailable
as a method
only for simple cases
and not for a problem.
But now imagine a problem
which has only one solution
even if its substrate is complex
then the resolution has
the level of a AHA-problem.
This scheme is a way to see
a small move with a big impact
like the last piece of a puzzle.