'With a knight on f5, the game often wins itself'.

You've probably heard the popular chess maxim in the title before. And for good reason. Garry Kasparov has been quoted as  saying that "a knight on f5 just about every time justifies a pawn sacrifice'!

I'd like to take you all back to July 2016. The strong Bilbao Masters Super-GM tournament is under way, and World Champion Magnus Carlsen faces off with the White pieces against the strong American Grandmaster Wesley So. 

Carlsen had started the tournament with a 'shock' loss to Hikaru Nakamura, his first Classical loss to him, but bounced back spectacularly to beat his challenger Sergey Karjakin and the Chinese prodigy Wei Yi in round 2 and 3. Now back in good form, how would he handle So, who finished 2nd in the strong 2016 US Chess Championship just 3 months prior?

Carlsen opens with Ruy Lopez, and So, perhaps trying to be solid against Carlsen, plays 3 ...Nf6, known as the Berlin Defense. This variation of the Ruy Lopez was popularized by Vladimir Kramnik, who used it as a rock solid drawing weapon against Kasparov in their 2000 World Chess Championship match. Carlsen rejects a possible transposition into an endgame, which could've been reached very early (as is typical in the Berlin) and plays 4. d3. 

After 12 moves we reach this critical position.

White now inserts the theme of this article and plays 13. Nf5!

Now you might wonder, can't the queen just scoot over to f7 and keep and eye on g7, After which he can castle into and equal position? 

If so, Carlsen's valiant knight on f5 has something to say about that!

So thus played the uncomfortable looking move 13. ...Qf8, preventing him from castling to safety.

A few moves later, in this position, you might think black could just win material and play Bxa3 and win white's A pawn. Again, the knight on f5 has other ideas!

Note how Black cannot simply kick the knight with g6?? as White always has the resource Nxd6 followed by g4 to win the bishop. And so throughout this game, the knight on f5 was a constant thorn on Black's side.

This position was then reached.

That knight continues to be White's key piece in the position, and restricts Black's options considerably. Black played b6 to protect the rook, but why not take the pawn on a4? 

And now how about in this position? What happens if So takes the rook on a1? 

What if black tried to activate his king rook by playing Kd7 and "protecting" the bishop?

So finds himself completely tied up with that mega-powerful knight on f5, which a)prevents him from castling due to the Bh6 idea mentioned earlier. b) severely restricts Black's options due to nasty tactics to win material.

Eventually So had had enough and resigned in move 26. Imagine, an 'early' resignation in the solid Berlin defense.

A knight on f5 is a fascinating attacking piece, and can provide the fulcrum for a both a strong attack and a restricting tool to limit the opponent's options.  Play it in your e4 games and win!