This is the conclusion of a famous trap in the Petroff defense. After the opening 1. e2-e4, e7-e5; 2. Nf3, Nf6; 3. Nxe5, Black should not reply with the symmetrical Nxe4. If he does, White plays Qe2—as happened here (look at the Black knight on f6; imagine it instead on e4, as it was a moment ago). The move seemed to attack Black’s knight on e4, but more importantly it put White’s queen onto the same file as White’s knight and Black’s king. The makings of a discovered attack thus were in place, with Black’s own knight on e4 the only impediment. Black blundered away the game by then moving his knight to safety with Nf6 (rather than guarding it with d7-d5), producing the result shown here; for this leaves the kernel of a discovery in front of Black’s king. White’s knight now can go on the attack with impunity. Black’s queen is within reach via Nc6+; the queen is lost to NxQ a move later, whether it stays where it is or moves to block the check on the e-file.