In this Lichess puzzle, after the initial moves
[title "Black to move"]
[fen ""]
[Startply "17"]
[StartFlipped "1"]
1. c4 b5 2. cxb5 Bb7 3. Nf3 g5 4. Nxg5 Bg7 5. Nf3 c5 6. Nc3 a6 7. d4 axb5 8. Nxb5 Qa5+ 9. Nc3 Bxf3!
Why can the bishop just take the knight? Why is it so bad for white to retake on f3?