Size up Black's threats. His rook is aimed at White’s knight, but the more arresting sight is Black’s queen and knight both trained on h2—and also on f2. Thus Black is close to a queen-and-knight mate with Qxh2, but the mating square is defended by White’s knight on f1. (Qf2 doesn't work because the king can retreat to h1.) Rather than being stopped by the sight of White's knight, your thinking should turn to ways to be rid of it. We saw that Black has the knight under attack, so suppose he takes it with RxN+; White has no choice but to capture the rook, as the king has no flight squares and there is no room to interpose anything. If White does it with QxR or BxR, the mating square has been freed up for Black to play Qxh2#. If White instead plays KxR, Qf2# now works because White’s king no longer can make it back into the corner. So Black's initial move RxN+ is conclusive.