The solver who starts by trying to construct a mating position will
discover that it only requires two black moves and two white moves. What
is the purpose of the mysterious white pawn?
(Remember that the move sequence in each solution is B W B W B W)
The two mating positions are king on e2 with rook blocking f1 and the
g3 knight checking from d4, and king on e3 with knight blocking f3 and
the g4 knight checking from d1. However, the king cannot make the
opening move, and it transpires that each time the only black piece
which is free to play is the piece that stands on the mating square.
Any move will leave a guard on the square, but this is neutralised by a
critical move followed by an interference by the white pawn.
1.Ra4 Sf5 2.Ke2 b4 3.Rf1 Sd4;
1.Ba4 Sf2 2.Ke3 b3 3.Sf3 Sd1.
Dafydd Johnston: Very nice to see a light and elegant helpmate.
I like the way the two solutions are matched using the alternative pawn
moves in the shut-off, which is only necessary because Black must lose a
tempo.
Peter Niehoff: The particular point here lies in the opening
moves which appear to act against the mate that both sides are striving
for. This AntiZielElement (anti-aim factor) compels White to compensate
for Black's damaging effect through line-closure by the Pb2.
Jacob Hoover: In each solution: a) Black's first move is a
critical move; b) White's response moves one of the knights away so that
the king can move to the square he's going to be mated on; c) White's
second move makes the mate possible by cutting off a black line (this
is the purpose of the white pawn!); and d) Black's third move is a
necessary selfblock that becomes possible only after the king moves.