PawnSac: THE LOSING MOMENT
< 39...c5? >
black does not count the squares accurately.
Before pushing the c pawn black must count FREE (unpaired) RUNNING moves to see who will queen first. Counting is simplified if you disregard PAIRED EXCHABNGES.
On the kingside...
White must make 6 free (running) pawn moves to queen. The first 3 to get his pawn to f6. Black must capture on f6 and white must recapture, so gxf and Kxf is a pair of uncounted moves.
The next 3 pawn moves are h6 h7 and h8=Q. So WHITE QUEENS IN 6
On the Queenside...
dxc Kxc, axb Kxb, a4 bxa4, thats 3 paired exchanges (uncounted). What we DO count is Kax4 Kb3 Kxc3; thats 3 moves, plus 4 pawn moves to queen. So BLACK QUEENS IN 7 (1 move late).
Since white will move first, he will use his 7th move to check, after which black must move the K (he can't queen).
The technique then is to keep checking with the Q and zig-zag down the board until immediately adjacent to the pawn, which black must guard by stepping in front of the pawn on the queening square. Now, if this was a C pawn and not a D pawn, black could move to a1 and after Qxc2 it would be stalemate. But alas, its a D pawn. Black loses.
Each time black must step onto the queening square, white uses that free move to advance his king.
Therefore, for better or worse, and the only drawing chance,
39. ...Ke6 was necessary here.