Should be Easy

This position is from the Anthology of Chess Combinations in the instructional section. I am using the electronic edition and the Chess Informant Solver's Kit. One encounters it with the knowledge that Black plays to reach a win. There's not much that Black can do, as most of his piece are immobile. Attacking the king--the only vulnerability--is the clear plan. Nonetheless, I struggled to find the correct move.

Black to move

I could see the idea that creates a checkmate threat. It does not work. How do I change the dynamics of the position so that Black's defensive resource is no longer available? Had I asked this question, which Paata Gaprindashvili suggests in Imagination in Chess (2004), the solution would have been immediately evident. Gaprindashvili uses the term "reciprocal thinking" for correcting an idea that does not work via altering the order of moves, or choosing an intermediate move.

Flexibility is another term that seems appropriate here. Despite a limited number of pieces, Black has more than one way to attack the king. Flexible thinking discloses the correct sequence. The term flexibility describes the resources on the chess board, and it describes the necessary mental processes for utilizing those resources.