Take this game:
[fen ""]
[Startply "13"]
1. d4 d5 2. Bf4 Nc6 3. Nf3 Bf5 4. e3 Nb4 5. Nh4?? Bxc2 6. Qd2 e5 7. Bg3 Be4 (7... Bxb1 8. Rxb1) 8. f3 Bxb1 9. Rxb1
I'm specifically curious about the difference between
- 7... Bxb1 8. Rxb1 and
- 7... Be4 8. f3 Bxb1 9. Rxb1.
Using Stockfish 14+ at depth 22/22, the engine thinks that the first option is -0.9 and that the second option is -2.6. The only difference between these two positions is the f pawn. So basically stockfish thinks 7...Be4 can "trick" white into pushing the f pawn to f3 which drastically weakens the position.
But I don't really see why the f3 pawn weakens white here.