Figure[Black to move]

Size up your capturing possibilities using the Black pieces. Your queen (and the bishop behind it) attacks the knight on c3; your queen alone attacks the knight on g5. The g5 knight is protected by the bishop at d2; the c3 knight is guarded by its queen and, again, by the bishop on d2. What to make of this? Neither of your captures looks productive, but that would change if the effectiveness of White’s bishop on d2 somehow were compromised. You can achieve that result by looking at the line between the bishop and the g5 knight and asking if you might disrupt it. A simple pawn push suggests itself: e4-e3, attacking the bishop and obstructing its path to g5. Black now wins a piece:

(a) If White takes the pawn with his bishop, his knight on c3 is left attacked twice and protected only once. Black wins it with QxN+.

(b) If White retreats his bishop (or plays f2xe3), his knight on g5 is left loose. Black can win it with QxN; but Qxc3+ is even better.

As noted a moment ago, when the e-pawn blocks the diagonal it also inflicts a threat against White’s bishop, and notice that the threat is crucial; if White had not been kept busy by it, he would have had time to move his g5 knight to safety. (This is one reason why f5-f4 would not have been comparably effective for Black, despite blocking the same diagonal. Another reason is that White then plays g3xf4, and now his pawn guards the knight.)