In the Spanish Chigorin system, Black had just played 27… h5, so it is White to move.
Even though most of the play is on the kingside, instead of retreating with the knight, John Nunn found a brilliant opportunity to utilize his open ‘a’ file -
28. Ra6!! - nicely exploiting the fact that g6 has been weakened with Black’s last move.
The game just lasted a couple more moves after Black accepted the sacrifice:
28... hxg4 29. Rxg6+ Ng7 30. Rxg7+ (30. Rxg7+ Kxg7 31. Bh6+ Kxh6 32. Qh4+ Kg7 33. Qh7#) 1-0
Replay the game in the viewer below: