Only two moves really come into consideration, here: ...Na6, to protect c7 directly; and ...e5 to mask the bishop's attack. The latter unfortunately loses a pawn: 4...e5?! 5.dxe5 a6 6.Nc4 fxe5 7.Bxe5 ...

5...Kf7 is surely the better retreat!

When that happens, always take note of what move the computer prefers. That makes it clearer whether you made a move allowing a strong response, or made an okay move but missed a better one.

White is satisfied with the win of the exchange and the break-up of Black's Q-side pawns - a winning advantage. But White could have preserved his rook advantage entire with 7.e4! Bxe4 8.Bxa6 bxa6 9.Qg4ch! ... (Even better than 9.Nc7 e5!, which wins back part of the material for Black) 9...f5 10.Qh3 Qxa8 11.0-0-0! ... and White will play 12.f3 to win the bishop trapped on e4. I'm pretty sure I would have played 9.Nc7?! in this line, which is good enough for a winning game, as indeed is White's actual choice.

Black was trying on the same trick as White played on him. White's defence is quite reasonable, but there was a better, if perhaps a little counter-intuitive: 8.Rc1! The threat is 9.a3 Na6 10.Bxa6 bxa6 11.Nc7. No good would then be 8...Nxa2 9.Ra1 Nb4 10.Rxa7 Nxc2ch 11.Kd2 ... with 12.Bd3 looming.

White has carelessly offered Black counterplay... good enough to draw the game!

17...Qc4ch! 18.Kb1 or Kd1 Qd3ch and a draw by repetition. having said that, I can see what Black might be after: opening the c-file for his remaining rook.

Mutual errors, here. I presume 18.Bc7 ... had in mind a follow-up based on 19.Qd6ch ... 20.Qd8ch... and 21.Qxh8, but really that would have been good for a draw at best. Black could have responded with 18...Ne5!, kiboshing the Qd6ch plan, forcing the Queen from its protective diagonal, and turning the tables completely on White. A possible line runs: 18...Ne5! 19.Qg4 h5! 20.Qh3 Rc8! 21.Rd1 Qc4ch with a winning attack. The move 18...Bd2ch is the third time Black overlooked the simple loss of material, but this time he had everything to play for.

He could not; 20... Kxc7 is met by 21. exf6 discovered check and 22. fxe7.

Well, for a wild moment Black had the chance for a straightforward draw, and then for a wilder moment, even a chance for a winning advantage. What that shows is that however great your apparent advantage, there is always a chance that a single mistake will throw it all away.