Some random chess problem I found

[mathgrant]

According to the site where I found it, this chess puzzle was created by a Martin van Essen, and published in 2005. I thought that some people on here might enjoy seeing it.

[L'lanmal]

I could wish for more clarity on what "Undoes a move" means. His own move? His opponent's? Any move all game? Half of an en-passant? Can undoing the move lead to an "illegal" position to move from? And whose move is it?

I assume it means the board shows a position immediately after white's move - show another possible move white could have made that would force black to mate him.

Anyways:
Undo f5xg6 (En-passent), then g2-g4. Only legal move for black is h4xg3 (en-passent)++

[mathgrant]

[MNOWAX]

that is the coolest chess puzzle that I have seen in a long time [/geek]

Nice find mathgrant!
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MNOWAX

[Courk]

I understand why black would make that move, what with winning being the point, but why is it the only legal move?

[Scurra]

It's taken me ages to work out why in my head. (That's why I gave up playing Chess!)

Remember that White has taken a move back. So there is now a White pawn on F5 but, critically, a black pawn on G5 as well - i.e. the Black King is hemmed in more than it appears from the diagram. This leaves Black with three possible moves - Kxf5, g5-g4 or f3xg2.
But one move allows White to block all of them, and simultaneously leaves Black with only one legal move - which is the checkmate. (Whereas just moving g2-g3 lets Black simply capture the white Pawn on f5 since it is undefended.)

At least I think that's how it works. I am generally rubbish at this sort of thing.
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[L'lanmal]

Yeah, it's the only legal move due to the absense of any other legal moves. There is nothing forcing that particular move (no check involved), just that all the black pawns are blockaded, and the king has no square it can move to.

This is the reason that it does not work for the white pawn to have come from h5, because then the black f-pawn could still advance, and black could avoid mating white.

[mathgrant]

[Courk]

Oh, duh, I forgot en passant means capturing a pawn, too.