White defends the f pawn with his Queen. White's Knight is preventing
the advance of the d pawn but the greater threat is Black moving his
Knight to h5 to make the Queen exchange at g6 and transfer of the h
pawn possible.  So White's 31st move prepares for a Knight exchange.
Black does not wish to give up his Knight but by moving his Rook to c5
he once again threatens the pawn push to d5. Both players are under
time pressure - one must complete 40 moves within the time limit or