A double-check with a knight and a rook: a turning point in the endgame
The White player chose to develop pieces and to hold the center of the board. This player developed the Queen to defend his positions and used a line of peons to make more room for the other pieces.
The 14th move is very important. Both players had 4 points each. But the White player commits a mistake leaving position f3 unprotected. The Black player identified a good opportunity to take advantage of that mistake: a knight rook against the King and the Queen! After capturing White's Queen, a material advantage was established.
After the Black's knight is captured, the Black player decides to attack at the center with the Queen. At this point, the White player had to rethink its strategy for the middle and endgame. The risk of loosing control of the center of the chessboard and/or loosing more pieces would imply in defeat. It was necessary to counter-attack.
The 17th move, therefore, is a critical point for the White player. At this point, White had two "free to move" rooks and two knights at the center, besides a lot of space for maneuvering. However, it was only a question of time for the black Queen to take control of the center or to create more damage.
The Black player fails to identify this turning point in White's plans. As evidence, on 17.Qc4, the Black player once again attack with the Queen in spite of developing other pieces to support her.
In the endgame, the White player decides to attack position e7 with one of the rooks. The Black player identifies another opportunity for a fork, this time a Queen's fork against rooks. This move could have set such a material difference, it could have been decisive. But, White had a trick to perform.
The White player moves one of the knights into position f6 and attacking black's King. However, this move uncovered a rook's check, as well. This is called a double-check and it is a very powerful move. The Black King's route of escape was limited by its own allies! The White player advances with the rook to position e8 and that is a Checkmate!
We can see how the material advantage could have decided the game, unless such an unexpected turning point in the endgame had taken place. The White player succeeded in transforming space advantage and superior development of pieces into an very aggressive move, revealing a change of tactics.
