White’s queen is loose and under attack, so as usual he considers whether to retreat or take the offensive. He could win Black’s queen if he first could be rid of the rook that protects it from d8, so of course he thinks about putting one of his own rooks on Black’s back rank. The trouble is that he can’t: his rook on e1 is aimed the right way, but the knight on e4 blocks its path. What to do? Move the knight, and do it violently so as to force Black to spend a move making a reply. A check is best, and a fork is best of all; so White plays Nf6+, attacking Black’s king and queen. After Black replies g7xN, the way is clear for White to play the traditional Re8+. Black’s king has no flight squares, so RxR is compulsory for him. Now White has turned the tables and plays QxQ; he has won a queen for a knight and a rook, a favorable trade. (Consider it as a matter of points. White converted the three-point advantage he had at the outset with his extra knight into a four-point advantage: now he has a queen and a rook against two rooks.)