This isn't the first time this move has been played, even in the chessgames database it has been played a few times before. Although now that it has been played a few times in games between grandmasters it's results are pretty staggering.

First, I would like to note that this moves strength surprises me not because it seems to force white to trade down material into a slightly worse endgame, but because of the passivity of this move. It is returning the knight to the same square it occupied before black's turn on move 2.

To contrast this with other moves that have been played on move 15. When faced with this same position in 2004, Topalov chose a more active route of castling. Vallejo-Pons vs Topalov, 2004

In this same position Wang Yue against Almasi also chose to castle on move 15. Z Almasi vs Wang Yue, 2006

There are other games with Karjakin and other grandmasters who have played this same position, and until recently the theory was to choose something more active like castling. However, white won slightly more games after the castle, even on the grandmaster level.

Later today, I want to look at what makes 15. ...Nb8 so much better than the more active options.

Okay, 15. ...Nb8 seems to break a lot of these rules, yet at least from the preliminary results it seems effective. First let's take stock of what this position looks like. White has seven pawns (with one doubled), two pieces developed (though without much of an attack to sustain), and perhaps a slightly larger area occupied by his pieces. Black has four pawns with one of them isolated. Furthermore he has three minor pieces, and neither of them has any real vulnerability.

If white can hold onto his pawns he has a pretty good chance in this game. Although, black has the bishop pair, and a lot of space to move his pieces, and also remember white's pieces are not coordinated, so it may take some time to create an effective attack.