If you play the French often enough, you have probably seen the …f6 break as a common theme to equalize space. One common example comes from the 3. Nd2 Tarrasch main line:
Here, Black can eliminate the cramping e5-pawn with 8…f6 with good piece play and open f-file, at the cost of a somewhat bad bishop and backward e-pawn.
However, there are many different variations and subvariations in which one can consider an …f6 break (in non-Tarrasch lines as well), and suffice to say that not all of them are good. Admittedly, I’m not an expert in the French, and I’m not sure how much studying one would have to do to cover all of these scenarios. My personal advice is to not break out …f6 when in doubt, since it does create weaknesses and in many cases can be delayed with few major consequences.
It’s easy to take the …f6 break for granted in lines like the above, but it does weaken Black’s center (two hanging pawns) and king. It (more or less) works for Black in the above Tarrasch because White’s pieces are not well-developed enough to take advantage of the weaknesses too soon and Black is positioned well to defend and even counter-attack due to the open lines created by …f6.
On the other hand, consider this position I recently happened upon from a Pittsburgh weekend tournament game between two experts:
Black played 7…f6 here. At first it doesn’t look too bad because the center is relatively closed and White hasn’t castled. But White’s pieces are much more developed here than in the first example, so attacking the d5/e6-pawns is a lot easier. Note that White hasn’t committed the light-squared bishop and still has the option of g3/Bh3.
That wasn’t Black’s only mistake, but it quickly made things more difficult for Black. After the natural 8. Qd2 a6 9. exf6 Qxf6 10. O-O-O Bd6 11. g3! b5?? (not what Black needs to be focusing on!) 12. dxc5 Bxc5 13. Bh3 Black’s position is virtually beyond repair. Although it doesn’t immediately work out tactically, White is already entertaining the idea of Nxd5 (as happened a few moves later).
Unfortunately, the rest of the game was not particularly hard to predict. White soon won both the e6 and d5 pawns, and still managed to attack Black’s weak kingside. This goes to show how seemingly insignificant differences can completely change the positional assessment of an …f6 break!