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In this game, white was on the move...

chessboard

FEN 8/k1n5/8/2K5/8/8/8/1R6 w - - 0 1

... and he played RB5+

This game was played in a tournament, and this move allowed him to get the first place. It was a standard tournament, with FIDE rules.

Why did white play this move, and how did it enable him to win the tournament?

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  • $\begingroup$ Quote from FIDE: III.5 If Article III.4 does not apply and the player having the move has less than two minutes left on his/her clock, he/she may claim a draw before his/her flag falls (see also Article 6.12.2). He/She shall summon the arbiter and may pause the chessclock. He/She may claim on the basis that his/her opponent cannot win by normal means, and/or that his/her opponent has been making no effort to win by normal means: $\endgroup$
    – Lucenaposition
    Commented Oct 23 at 22:38

3 Answers 3

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White doesn't have enough time on their clock to close out the game.

They need at least a draw to win the tournament and if they flagged now they would lose as there's a sequence of moves by which black could still checkmate. Rb5+ ensures that either white loses both their rook and then their pawn after c4+, or black loses their knight. Either result means that black can no longer checkmate, even with white's help, causing the game to draw if white ran out of time.

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    $\begingroup$ This answer could be improved by showing the theoretical positions in which black win if white do not lose the rook/pawn. $\endgroup$
    – Andrew Savinykh
    Commented Oct 25 at 3:47
  • $\begingroup$ I don't think there's a king-knight checkmate possible without cooperation from the other player. His strategy was to force a D before his clock ran out, giving him the L. Black doesn't have a possible checkmate here, but white does with the rook--and enough time. $\endgroup$
    – Cristobol Polychronopolis
    Commented Oct 25 at 15:21
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    $\begingroup$ Black does have a possible checkmate here, it's just also a helpmate. IIRC the FIDE rules require that it not be possible at all for an opponent to checkmate in order for a flag to be downgraded to a draw. $\endgroup$
    – GoblinGuide
    Commented Oct 25 at 16:21
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Perhaps

He only needed a draw to clinch first.

After

1... Kxb5 2. c4+ Kxc4, it is a draw due to insuficcient mating material for each side

If

black doesnt capture the rook (or the pawn on the second move), then white takes the knight and black has only a king and can't win.

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  • $\begingroup$ Good start, but there is a reason why white had to play this and no other move. The question states "this move allowed him to get the first place" explicitly. From this it can be derived that any other move would have resulted in losing the tournament. $\endgroup$
    – vsz
    Commented Oct 23 at 15:05
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    $\begingroup$ White only had time on the clock to make 1 move and optionally take knight then, other explanation is ok from @SteveV. $\endgroup$
    – z100
    Commented Oct 23 at 16:39
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Perhaps

White only had time to play 1 more move

and

the move White played resulted in threefold repetition to immediately secure a draw.

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  • $\begingroup$ Also possible, but in this case puzzle is not as good as it should be. $\endgroup$
    – z100
    Commented Oct 23 at 16:58
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    $\begingroup$ I don't see why this answer should be less valid than the accepted one ... It even works under much more drastic time trouble $\endgroup$
    – Tim Seifert
    Commented Oct 24 at 12:00
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    $\begingroup$ But what's the likelyhood that white would have gotten away with moving their rook there three times in a row? It doesn't matter where black has their knight, black would certainly take with king. $\endgroup$
    – Robin Clower
    Commented Oct 24 at 17:19

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