This time White’s queen is the piece that is cramped: it’s up against the side of the board with little room to retreat. Black considers a threat against it with g6-g5. Where will the queen go? It gets captured if it moves to h5 or to anywhere on the fourth rank (look and see). So it has to move to h3. Yet now its relationship with the knight on f3 calls for a pawn fork: g5-g4 wins the knight, with Black’s own knight at f6 supplying the needed protection for the pawn.
In the game this position came from, between Tal and Botvinnik, it was White’s turn to move. Tal played Nc3-d5. Now if Black plays g6-g5, White plays NxNf6+. Since this checks the king, Black can’t play g5xQ; he has to capture White’s knight. After that exchange gets rid of the knight on f6, White’s queen has plenty of flight squares and the fire is out. But of course the first important point in all this is for White to recognize that he is in danger. The tipoff is the immobility of his queen.