White could play 19.Rd2 and offer a draw.

But 19.Qg4?? and Black is "throwing crap at the Queen until she's forced off the Rook".

The back rank mate beckoned. Deflect the white queen then take the Rd1 with check and mate. How to deflect the white queen? h5 doesn't work, because white can just take the pawn and stay on the d1-h5 diagonal.

So the starting move has to be 19...f5, when 20. ef brings our f8 rook into the attack.

Can white play anything to counterattack? How about 20. Rd2 to attack the white queen and wriggle out of the back rank mate threat? Nope, that doesn't work. 20...fxg4 21. Rxc2 Rxc2 and we are a rook up.

Can't see anything else good for white, so our starting sequence should be 19...f5 20. ef.

Now what? We bring the f8 rook into the game with irresistible threats against both f2 and Rd1. So 21...Rxf5 and doesn't that seem a little tweazy for a saturday?

What I had missed, of course, are two moves that didn't work one move earlier. h5 was no good at move 19, but absolutely a killer on move 20. And white's defence Rd2 was insufficient at move 20, but worked at move 21. I think I had categorised both moves as "don't work" and mentally filed them away.

Lazy, lazy, lazy.

Ulibin vs Sveshnikov, 1988 (19…?)

Black to play and win.

Material: Even. The White Kg1 has 2 legal moves, but is potentially vulnerable to back-rank mates. Black has a battery Rc8 and Qc2. The White development is incomplete. In particular, Bc1 disconnects Ra1 and Rd1, so Ra1 is unprotected and Qg4 has the absolute burden of preventing …Qxd1# . The Black Rf8 and Be7 require activation. By enumeration, the candidate is neither a check nor a capture.

Candidates (19…): f5

19…f5 (threatening 20…fxe4 winning a P)

The move …f5 is often thematic for the Sveshnikov Sicilian, which the puzzle position resembles. The White Qg4 has the burden of preventing mate, so this move hurts. Over the board, positional considerations justify the candidate, so a tactical analysis is required only to note that it drops no material.

20.exf5 [else, drop at least a P]

20…h5

The move 20…Rxf5 is good, but analysis shows it insufficiently forcing to win. The zwischenzug 20…h5 permits the capture …Rxf5 with tempo, because of an attack on the White Q. White can either capture Ph5 or not, but the White Q must maintain protection against …Qxd1#.

(1) 21.Qxh5 Rxf5

(threatening 22…Qxf2+ 23.Kh1 Qf1+ 24.Rxf1 Rxf1# and 22…Rxh5)

White drops at least a R (or Q for R) to prevent mate.

(2) 21.Qf3 Rxf5

The same threat ends the game.

[ply 15/36 time 00:05 value (to White) +1.30]

19...f5 20.exf5 <Rxf5> 21.Rd2 Qxc1+ 22.Rxc1 Rxc1+ 23.Rd1 h5 24.Qe2 Rxd1+ 25.Qxd1 e4 26.Qd4 Re5 27.Kf1 Kf7 28.Ke2 Bg5 29.h3 a5 30.a3 Kg6

With humans able to improve near the end of the full computer best play variation, Toga II 1.3.1 gives

[ply 15/47 time 00:10 value (to White) -5.90]

19...f5 20.exf5 h5 21.Qf3 Rxf5 22.<Qd3> Qxf2+ 23.Kh1 Rc3 24.Be3 Rxd3 25.Bxf2 Rxd1+ 26.Rxd1 Rxf2 27.a3 Bg5 28.Kg1 Be3 29.Kh1 e4 30.h3 Rb2 31.b4 Ra2 32.Rf1 Rxa3 33.Kh2 Ra2

<19... f5
20. exf5 Rxf5
21. Bh6! Qxf2+
22. Kh1 Rf7
23. Rg1 Rc2 >

In this sequence, after 22...Rf7?? White just plays 23.Qxc8+ and wins.

Best for Black on move 22 is perhaps 22...g6, when he has some advantage.

21..e4! is even stronger but I think 19..f5 and 20..h5 are the key ideas.

It depends on the position and the clock. GMs will play on a piece down in some circumstances - eg if they have a good position or their opponent is short of time. Of course, good players will sometimes sacrifice material for positional gains, so being a piece down is not an automatic loss.

But here white is not only material down but in a worse position. As <DevastatioN> says, black will probably play 23...Qxf2+ first before snaffling the Bg5. Black has well posted pieces, command of both open files (c and f), queen on the seventh rank and a passed pawn. The black bishop will own several juicy diagonals, including the long black diagonal. White has an offside queen, rooks useless on the back rank and a cornered king.

So yes it does look like a good time for white to resign.

The immediate f5 is the better move over h5 for three reasons. 1. It is a pawn advance that is well protected. 2. It is forcing since it forks the queen and e4 pawn, and White cannot afford to lose the e4 pawn at this stage of the game. 3. It will activate the f-file rook.

19. ... f5 20. exf5 (giving Black the central passed pawn) h5! is again best because it immediately attacks White's queen. (20. ... Rxf5? lets White off the hook by losing the initiative allowing 21. Rd2.) If 21. Qxh5 Rxf5 22. Rd2 Rxh5 23. Rxc2 Rxc2 with a winning game for White.

White really has no choice but to play 21. Qf3 hoping to get to d3. But, 21. ... e4 keeps the pressure on White and attacks d3, the last safe haven for White's queen. Rxf5 doesn't work here for the same as it didn't on move 20.

22. Qxh5 (last ditch effort) Rxf5 seals the game. Nicely done by Black.

<beginner64 Where exactly does white lose it?>

White's position begins to slip after 9. Be2?. Stronger moves were the immediate aggressive Nd5 (a move that isn't all that troubling for Black), Nc2 or Bd3 since they both prevent Nd4. Black then does a nice job of pressuring the exposed c4 pawn. White's mistake here is failing to recognize that he can't hold the c4 pawn. He should have worked to defend the knight on c3 with 14. Bd2. Rd1 followed by Ned5 looks promising, but opening up the queen side before developing those pieces is generally unwise.

<Was 19. Qg4 a double question mark?> Yes. After the exchanges and Black's queen arriving on c2, White should have exchanged queens. The position would have been slightly in favor of Black, but far from clear with a draw likely.

A) 20.Qf3 fxe4 21.Qg4 Qxf2+ 22.Kh1 Qf1+ 23.Rxf1 Rxf1 mate.

B) 20.Qh5 g6 21.Qf3 fxe4 as in line A).

C) 20.exf5 h5

C.1) 21.Qxh5 Rxf5 22.Rd2 (22.Qg4 Qxf2+ 23.Kh1 Qf1+) Rxh5 23.Rxc2 Rxc2 winning.

C.2) 21.Qf3 Rxf5 22.Rd2 Qxc1+ 23.Rxc1 Rxc1+ 24.Rd1 Rxf3 25.Rxc1 Rd3 with a won endgame.