Fundamentals (part 15)

D Scott (166) - Spanton (147)
Hastings Main A 1990

Several candidate moves come to mind, including ...c5, ...b5, ...a6 and ...Ke7, but can you tell which one of them loses?

*****
*****
*****
*****
*****
*****
*****
*****
*****
*****
32...c5?
This loses, as will be seen. The other three moves draw, but it is easier to understand why by first seeing how the text loses.
33.c3 c4
White offered a draw.
34.a4 ½–½
White should not have offered a draw. After 34...a5 (34...a6 is met in the same way), White has 35.g5 fxg5+ 36.Kxg5 and Black will lose the d5 pawn, eg 36...Ke7 37.Kf5 Kd6 38.Kf6 Kd7 39.Ke5 Kc6 40.Ke6 etc.
So the reason 32...c5? loses is that it unprotects the d5 pawn, after which the black king is shouldered out of the way by the white king The same is not true after the other three suggested moves, although Black has to beware zugzwang, eg:
A) 32...b5 33.b4 Ke7! (an only-move, as will be shown later) 34.g5 fxg5+ 35.Kxg5 Ke6 (another only-move, but this time easier to find) 36.Kg6 Ke7 (only-move) 37.Kf5 Kf7 (only-move) 38.Ke5 Ke7 (only-move) 39.c3 Kd7 (39...a6 also draws) 40.Kf6 Kd6 (another only-move, but now it can be seen White cannot make further progress as the d5 pawn is defended and he cannot get at the c6 pawn).
Going back to the position after 33.b4, the plausible 33...a6 loses to 34.g5 fxg5+ 35.Kxg5 because White has two reserve tempi, eg 35...Ke7 36.Kf5 Kf7 37.Ke5 Ke7 38.c3 (the first reserve tempo) Kd7 39.Kf6 Kd6 40.a3 (the second reserve tempo) Kd7 41.Kf7 Kd6 42.Ke8 Kc7 (or 42...Ke6 43.Kd8 Kd6 44.Kc8 etc) 43.Ke7 Kc8 44.Kd6 Kb7 45.Kd7 Kb6 46.Kc8 Ka7 47.Kc7 and the c6 pawn falls.
B) 32...a6 33.g5 fxg5+ 34.Kxg5 a5 35.a4 Ke7 (an only-move as Black needs to keep the d5 pawn protected from c6) 36.Kf5 Kf7 37.Ke5 Ke7 38.c3 (White uses a reserve tempo so he can shoulder the black king out of position, but here it is not enough to win, although Black has to continue to find only-moves) Kd7 39.Kf6 Kd6 40.b3 (not 40.b4? as 40...b5! wins since Black will get an unstoppable passer) Kd7!? (alos drawing is 40...b5 and, now that b3 has been played, even 40...c5, but the text sets a trap) 41.Kf7 Kd6 and White has to accept a draw as the analysis engines Stockfish10 and Komodo10 show 42.Ke8? loses to 42...c5!, eg 43.Kd8 c4 44.bxc4 dxc4 45.Kc8 Kc6 with ...b5 to come.
C) 32...Ke7 (perhaps surprisingly, at least at first glance, the engines reckon this retreat also draws) 33.Kf5 Kf7 34.a3 a6 35.c3 b5 36.b4 (Black has been run out of pawn moves and has to move his king, but White has no reserve tempi and so cannot win) Ke7 37.g5 fxg5 38.Kxg5 Kf7 (38...Kd7 also draws, but losing is 38...Ke6? 39.Kg6 Ke7 40.Kg7! Ke6 41.Kf8 as the c6 pawn eventually falls) 39.Kf5 Ke7 40.Ke5 Kd7 41.Kf6 Kd6 - Black draws as his king cannot be shouldered out of the way by the white king.