Now let's see how it can look when a knight discovery involves a threat of mate. In this case the discovery isn't White's best move, but let's find it anyway. White's knight masks his queen: the kernel of a discovery, even if it's a little hard to notice because the queen appears to have no target on f8. But you always want to be aware of the enemy king and its constraints; it's especially important when you have a piece aimed at the back rank. What happens if White moves the knight and then plays Qf8 a move later? It’s mate. So White has a perfect target for the queen after all in the f8 square. He can play Nc6; and after Black fends off the threat of Qf8#, White has NxQ.
What is White's best move? Qe5+; Black has to reply with f7-f6, which then results in Qxf6#.