I just got off FICS and this was a win against 1500 rated player.
We arrive at this position on move 10 as shown. I am Black. White had just played 10. a3 (see diagram on left).
What should Black do in this position? I thought over this for a long while. Black has a terrible weakness at e5, so I must secure it first and take control of the b8-h2 diagonal.
10...Qc7 was the natural move. I could have considered Bd6 as well but I'm not worried about White's response Bf4 or Ne5.
11. b4? (see diagram on right. I thought this move was premature as it allows me now to gain the initiative) b5 (stopping counterplay on the Queenside)
12. Be3? (a weak move but where else can White's Bishop go?) Bd6 (continuing development)
13. Qc1 (doubling on the dark square diagonal to try and take control of the dark squares)
I paused to consider Black's response for a possible dark-square invasion by White but I could none. Bg5 aims at nothing and my Knights are doing a fine job protecting each other.
However, I smell blood and immediately hit on the a-file break with
14. Ne2 axb4
15. axb4 Bxb4 (winning a pawn)
16. c3 Bd6
17. Rxa8 Rxa8 (it does not look like it but White is now in a very dangerous position. I control the a-file and White tries to break down my kingside)
18. Bf4 Bxf4 (the more minor pieces coming off, the stronger my passed b-pawn becomes)
19. Qxf4? (this was unnecessary) Qxf4 (exchanging all the pieces)
20. Nxf4 (see diagram on left)
This line makes me think the longest. There are 2 possible problems Black has here.
I first consider 21. Ne5. This move simply forces me to recapture with ....Nxe5 22. dxe5 and my response Nd7 (threatening the e5 pawn) is good for Black.
So that leaves White with another likely response and that is to play 21. Nxe6 fxe6 22. Rxe6 winning the e-pawn and threatening my backward c5 pawn. I must find a way to stop this.
I've got a few ways of defending this.
I first looked at the defense of the c5 pawn with my Knight 22... Ndb8. I thought this was a very ugly move. If 23. Re7! My goose is cooked as I've no way of defending with a Rook on the seventh rank and no way to extricate my Knight.
I think to myself, maybe I can play Re8 (defending the Bishop) or Rc6 or even Ra6. However, I was extremely unhappy with the passive nature of the Rook. Surely there must be something better.
What weaknesses does White have? Very easy. The c3 pawn and the Bishop on d3. And to tie up things even more nicely, once my Rook takes on the pawn at c3, it protects the c6 pawn. Nice!
So I played 20... Ra3 (see diagram on left).
White as expected eliminates my Bishop and now, Black is winning.
21. Nxe6 fxe6
22. Bf1 Ne4 (now I just steamroll White's position with my b-pawn and White resigns shortly after).
23. Ne5 Nxe5
24. dxe5 Rxcee 3
25. Ra1 b4 (see diagram on right)
A nice win. IMHO I think White's mistakes were:
a. he was slow to react
b. he started trading off pieces.
c. rather than activating his pieces, his pieces started to withdraw.
d. the b4 pawn move was premature and in the end, it proved costly for White.