The f3 Nimzo is a very sharp and very complicated way for white to try to dominate the center.
For the basics of the opening and plans and ideas for both sides, watch this introductory video: https://youtu.be/xX_bnk7nKNg
The basic struggle which can be seen in every Nimzo Indian variation is the fight over the e4 square. White will go to extreme lengths to push e4 through, and black will do even more to try and prevent that. That is what makes the move f3 extremely logical and appealing for white.
F3 (known as the Kmoch variation) is a move in the nature of the position, but it is also double edged and fairly risky. Same as Qc2, which prevents black from doubling white’s pawns, it comes with a clear downside. It’s not doing anything for white’s development or king safety. In fact, it makes it harder to develop now since the g1 knight cannot go to f3 anymore. White is fighting for the center but risking a quick attack on his e1 king.
Black has to play actively to exploit that. Practice has shown that white can get away with the move f3. There seems to be no straightforward way to punish the move. Black can choose three different approaches to the position, some more and some less aggressive, but, if white knows what he is doing, black cannot get more than equality.
Definitely a worthy variation to study!
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