White has just pinned Black’s knight with his bishop on g5. How should Black respond? This is a common sort of pin, and when the bishop that inflicts it is unguarded, as it is here, it is dangerous for White. Notice that it sets up the kernel of a discovered attack against the pinning bishop; if the knight can move out of the way with check, Black will be able to play QxB a move later. You will want to be careful about so leaving pieces loose even when they are inflicting pins. In this case it might not seem like a problem because the pinned knight cannot reach White’s king to give check. Yet. But Black looks for any checks he can give and finds this one: Bxf2. White presumably replies KxB (we will consider another option in a minute). This moves his king forward a rank, which is far enough. Now Black plays Ng4+, checking White’s king. The king must move, and then Black plays QxB and collects the bishop. The pin is broken, Black gains a pawn, and White is on the defensive and has a king that cannot castle.

Now a few afterthoughts:

(a) When Black uses his knight to attack White’s king, notice the importance of playing Ng4+ rather than Ne4+. The latter move allows White to reply Ke3 and then KxN next move; for if Black’s knight is on e4 it has no protection after Black plays QxB. If the knight instead is on g4, then Black’s capture QxB also gives the knight a guard and makes it impervious to attack by White’s king. It might seem Black could solve this by playing the knight to e4 and then using the knight (rather than the queen) to take White's bishop on g5; but then White plays h2-h4, and Black's knight suddenly has no safe place to go.

(b) You might wonder what happens if White replies to Bxf2 with Kd2 rather than KxB. He gives up the piece that he otherwise would have taken; does he get it back? No. It is true that now Black is forced to check with Ne4 rather than Ng4, and that the knight still has no protection on e4. So White can reply to Ne4+ with Kd3 and then reply to QxB with KxN. But it’s costly: Black plays Qe3+, where the queen takes protection from the bishop now on f2. Now White’s only legal move is Kf5, which Black meets with a delightful discovered checkmate: d7-d6#. It will take a moment to see how this works and to realize that White’s king has nowhere to go at the end. It’s worth it. A moral: venturing your king out onto the board early in the game is a hazardous business.

(c) Black has a different sequence in this position that also works to break the pin. He can simply play Ne4. Now if White plays BxQ, Black mates with Bxf2#. So instead of BxQ, White instead has to retreat his bishop to e3. Now Black plays BxB; White replies f2xB; Black has Qh4+; and White is forced to interpose with g2-g3, leading to further complications and leaving his pawn cover in unfortunate shape.