Polerio defense 8.Bd3 line

I was studying polerio defense, possible 8th moves are 8.Bd3, 8.Be2, and 8.Qf3. I was studying the 8.Bd3 line, and I found that the mainline of this is

1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5 d5 5. exd5 Na5 6. Bb5+ c6 7. dxc6 bxc6 8. Bd3 Nd5 9. Nf3 Bd6 10. O-O O-O 11. Re1 f5.
   Black is a pawn down, but he gives one more pawn. And later white gives up the knight. Im 1200 rated, can anyone explain this to me? What is the purpose of f5 and what happens if white tries to save the knight?

Here is a flawless ICCF correspondence game with it

@tpr said ^

Here is a flawless ICCF correspondence game with it

https://lichess.org/game/JoN05n8E#22

I already checked the line after f5, but I don't understand the idea of f5 and why white gives up knight and what happens if they keep it. Can you explain it with words, please?(I'm bad at chess lol)

"I don't understand the idea of f5"

• Black is a pawn down, do he must attack to realize dynamic compensation otherwise he goes to a lost endgame.

"why white gives up knight"

• White does not really give up his knight: white regains the piece with the double attack 18 Be4.

@tpr said ^

"I don't understand the idea of f5"

• Black is a pawn down, do he must attack to realize dynamic compensation otherwise he goes to a lost endgame.

"why white gives up knight"

• White does not really give up his knight: white regains the piece with the double attack 18 Be4.

What about 14.c4 Nf4 15.Bf1 g4 16.d4 gxf3? This line is actually more common than 14.g3.

"What about 14.c4 Nf4 15.Bf1 g4 16.d4 gxf3?"

• It is equal just the same:
  
  https://lichess.org/game/WPWnT6sb#27

Those lines can be a little scary for white, that's why I recommend 10. Nc3 instead of 10. 0-0

https://youtu.be/6vOdv6UZUcc

Regardless the f5 lines are scary & should be studied, I lost one recently when black played f5 in a slightly different position (

I shouldn't have played 16. d4?, white should be slightly better if he hunkers down & defends but it's very easy to mess up & get into a much worse position.

Black's f5 tries are probably black's best attempt for counterplay in the Two Knights & it's a good idea to mess around with the engine & databases checking out different lines.

Yes, 10 Nc3 is interesting too, still dynamic equilibrium: