Black’s king is trapped; it can't escape to g6 if it's pressured from behind. A natural idea for White thus is Qe8, preparing for the kill on h8—but Black’s bishop guards the square, so White has to look elsewhere for ideas. Start by examining every check. There is just one: Rh8. And Black has only one reply to this: KxR. Now reexamine how the board will look. White’s knight and Black’s king and queen all will be on dark squares, inviting the fork Ng6+. You could have arrived at the same conclusion by noticing that White can attack Black’s queen with Ng6 and asking whether there is any way to draw Black’s king onto the square at the other end of the fork, h8. A common way to move an enemy king onto a square where you want it is to put one of your pieces on the square, attacking the king and requiring it to move onto the square with a capture to defend itself—a decoy. Here the White rook can do the job.