What made it so exceptional is the circus move 19 Nc4 leading not to some decision either way, but to a further positional exchange sacrifice on move 22 which was unclear for 30 moves.

Here is an introduction to this incredible game in the detailed analysis by GMs as given in a book on Sanluis 2005 by Alik Gershon, Igor Stohl :

< Topalov vs Anand featured a uniquely dramatic battle. The Bulgarian faced one of his main contenders for the title. He chose a relatively quiet line of the Queen's Indian. This choice was especially surprising as not long ago Topalov, against the same opponent, opted for a much riskier approach, emerging victorious after a brilliant game. The initial stage of the game was well known to Anand, who is not only the World's leading expert on the black side of this line, but has also employed this variation with White on numerous occasions. He, therefore, chose the safest (to his mind) continuation. The point of which is that White is practically forced into sacrificing an exchange for the initiative, but black has good chances to withstand it.

Topalov stepped up to the challenge with great enthusiasm, uncorking a novelty in his favourite style- for a fight. From here on it was one big illusion, as many spectators mistakenly counted the game as all Topalov's achievement, alloying Anand the role of a supporting actor. In reality, Anand reacted well and could have forced a draw. It was due to his doubts (on two occasions , as the situation was repeated later in the game) whether it was worth playing for a win and how to do it, that he erred and found himself in difficult positions, and even a lost one in the ending.

Topalov has to be credited for exploiting his chances, and for a correct psychological line: Anand was tearing himself apart between choosing a draw (from a superior position) with black or taking risks while playing for a win.