It’s Black’s turn to play, and his queen is attacked by the rook at a1 and the pawn now on b4. It would be natural but erroneous for him to begin looking for places to move the queen. Black has a well-advanced knight at d4 and so should be looking for forking chances. The knight can attack the White king in one move to f3 or c2; and at c2 it can fork king and rook. The only trouble is that the queen guards the needed square. Is the queen protecting anything that Black can attack? Yes—the rook at a1. So Black plays 1. ...QxR, which is met with 2. QxQ—now making possible 2. ...Nxc2+. Black nets a rook and a pawn. One lesson of the position is that when you are under attack, sometimes the best defense is a good offense: consider a capture rather than a retreat, and ask what the board would look like afterwards.