Why did this guy resign, was there a strategic reason?

Question

I have a hobby as an amateur chess player to watch top rated games on lichess.org, my personal favorite online chess platform. Usually after some analysis, I can figure out why players do particular things. However, with this game there really seemed to be no reason for the resignation. Did the player give up for non strategic reasons or am I missing something?

[FEN ""]
[Event "Rated game"]
[Site "http://lichess.org/cg4yGry4"]
[Date "2016.03.25"]
[White "LadyBaba"]
[Black "BlandPassivity"]
[Result "1-0"]
[WhiteElo "2223"]
[BlackElo "2121"]
[PlyCount "39"]
[Variant "Standard"]
[TimeControl "600+0"]
[ECO "A02"]
[Opening "Bird Opening"]
[Termination "Normal"]
[Annotator "lichess.org"]

1. f4 { A02 Bird Opening } d6 2. Nf3 Nf6 3. e3 Nbd7 4. a3 e5 5. fxe5 dxe5 6. Nc3 Be7 7. d4 exd4 8. exd4 O-O 9. Bd3 b6 10. O-O Bb7 11. Qe1 c5 12. Qg3 cxd4 13. Nxd4 Bc5 14. Be3 Re8 15. b4 Bxd4?! { (0.56 → 1.18) Inaccuracy. Best move was Bd6. } (15... Bd6 16. Qxd6 Rxe3 17. Qf4 Re8 18. Rae1 Rxe1 19. Rxe1 Nh5 20. Qe3 g6 21. Ndb5 Ndf6 22. Qe7) 16. Bxd4 Rc8?! { (1.17 → 1.84) Inaccuracy. Best move was Qb8. } (16... Qb8 17. Qh3) 17. Rad1 Qc7 18. Qh4 h6 19. Nb5 Qc6 20. Rf3 { Black resigns } 1-0

Answer 1

White's threats revolve around Nxa7. What can Black do? This is the line given by Ubisoft's Chessmaster:

2r1r1k1/pb1n1pp1/1pq2n1p/1N6/1P1B3Q/P2B1R2/2P3PP/3R2K1 b - - 0 1

1... Ra8 {Directly defending a7} 2. Rg3 {Threatening to take on h6} Kf8 {Breaking the pin} 3. Qf4 Nh5 {This forces white into the sacrificial line that follows} 4. Bxg7+ Nxg7 5. Nd6 {Threatens 6. Qxf7#} Ne5 6. Qxh6 {Sure, Black has a piece for 2 pawns, but White's attack is too strong}

Answer 2

White has a couple of good strategic ideas here, and Black's position makes it hard to combat them:

• After Rd2 (or Rf3), Nxa7 forks the queen and bishop, so Black's attempt to create counterplay with ...Qc6 has just gotten her into more hot water
• Qf4 will support Nd6, which forks the rooks if they stay where they are
• Qf4 also pins the knight to f7
• Black can try driving off the queen with ...g5 and ...Nh5, but she just goes to Qf2 and the f7 square is hit again (while the knight has been lured out of position to the sidelines)
• After the tussle's over between the knight and the queen on the kingside, which Black loses a tempo resolving, White plays N(d6)xe8, winning the Exchange
• White then simplifies to a BvN endgame where his bishop is superior to Black's remaining knight, and exploits his queenside pawn majority to create a passed pawn, diverts the black king to the queenside, mops up the black kingside pawns with his king, promotes and mates. Easy-peasy.

Answer 3

Black has an inferior, and theoretically lost game. On the other hand, it takes a certain level of skill for White to realize his advantage and actually win the game. White's advantage is that his b knight threatens a bunch of forks on the queenside, and that he also has a kingside attack. That's why he is winning.

If the players are both Grandmasters, Black will realize that he took a "wrong turn" earlier, drifted into a clearly inferior position, and understand that White is strong enough to take advantage of this fact. Therefore, s/he will resign.

On the other hand, this position is not clearly won for White if both players' ranks are in the low 2000s. At the level, Black will do well to play it out and see what happens. White may build on, and realize his advantage, in which case Black should resign. Or White might make a "slack" move and give Black a chance to catch up, or even take the lead. In this kind of situation, one move could make all the difference, and Black should force White to make "ten out of ten" correct moves in order to win.