Black just played Bc5, adding a second attacker against f2 along with his knight; maybe he meant to threaten a knight fork there. To see why this was a mistake, look at the board from White’s standpoint and ask some standard questions: What mating threat can White make? His bishop attacks f7, a square next to the king and unprotected by any piece other than the king; so White asks whether his queen can add to the pressure against that square and attack any loose Black pieces at the same time. The answers are yes and yes: if White plays Qd5, he now has a mating threat; and he also attacks the loose Black knight at e4. The knight can jump out of the way and help guard f7 with Ng5, but it's no escape. White already has g5 under attack twice.

So after 1. Qd5 Black is obliged to lose material. He can play the check Bxf2+, so that his bishop takes a pawn with it on the way down; but then White calmly plays Ke2, and now two of Black's pieces are under attack—his e4 knight by White’s queen, and his bishop by White’s king. He will lose the knight next move.

Lesson: leaving loose pieces around is dangerous business. Before putting his bishop on c5, Black should have noticed (a) that he already had a loose piece in the middle of the board, which always is cause for concern; and (b) that White had a bishop attacking f7, which is cause for additional concern since it sets up a mating threat if White can aim his queen at the square as well. Black would have been better off making a move to address one of those threats. Best is d7-d6, since then after 1. Qd5, Be6; 2. QxN, d6-d5 Black wins back his piece with a pawn fork. (We will look at pawn forks more closely in the later chapter dedicated to them.)