Figure[Black to move]

Black’s knight masks his queen. The queen doesn't yet have a good target, but it's aimed at the pawn in front of Black’s king; so Black might have a discovered check if the position were jiggered slightly. Consider what other pieces on both sides factor into the position. Black’s rook also attacks the c2 pawn. White’s queen is nearby, at d1. An idea comes into view: Black's knight could attack the queen with Nf2, and unmask a discovered check at the same time, if the pawn on c2 were removed; and perhaps the rook can perform the removal with Rxc2. If White responds with KxR, then Nf2 indeed wins the queen (whether the queen stays where it is or interposes on d3).

Okay, but what if White instead replies to Rxc2 with QxR? Answer this question in standard fashion: examine every check you would be able to give in return. Black's only check here would be Qe1. No moves the White king could make would save it; White would have to interpose his queen, and whether he does it on c1 or d1, Black mates: QxQ#.