10

I was looking at the answer of today's puzzle in chessgames and see that the reason I wasn't able to solve it because I missed a key move in the solution:

Black played 49...Rb4+
note: this is not the puzzle position, this was taken from the middle of the puzzle solution.
note2: the board seems to be correctly flipped.

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49 

I don't really see the advantage of doing that move, is it something related to winning tempos? If so, how?

  • 1
    I think I got it, the pawn will give check if white tries to queen too (e.g rook check, king moves, rook to back rank, white pushes pawn, RxR, white promotes, black promotes with check, game is over), but I don't think I should delete this question, maybe someone finds it interesting or helps this site survive the beta, who knows – Gaph Apr 5 '13 at 18:07
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Moving the king once like that is a priming move in order for Black to be properly aligned at a crucial point in a few moves.

Without the priming move this happens which allows White to win

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49

1... Rb1 2. d7 Rxe1 3. d8=Q c1=Q 4. Qf8 Kh5 5. Bf3++

So it is desirable to get a check in when promoting in order to win the game:

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49

1... Rb4 2. Ka3 Rb1 3. d7 Rxe1 4. d8=Q c1=Q+ 5. Kb3 Re3+ 6. Qd3 Rxd3+ 7. Ka2 Ra3++

I decided to show a simple mate ending. It is possible for white to prolong the inevitable . This would be done with 3. Be4 . Although it is possible for white to slightly delay, Black still wins with Rb1.

3

Forcing the white king to step forward and allow him to promote with check keeps white from drawing by perpetual check. If 49. .. Rb1 then 50.d7..Rxe5 51.d8=Q..c1=Q 52.Qf8+..Kh5 53.Qf5+..Kh6 and the last two moves would be repeated making a draw. With the white king one step forward and the check with the pawn promotion white can never get the queen over to check the king since blacks rook and queen can keep the white king in check every move as the net is tightened.

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