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Problem Observer, 1993

#2
GRASSHOPPERS a7, e8, h6;
b2, b3

1.Ke1! (2.Bg5#)

1...g5  2.Qf4#
1...Re5 2.Qe4#
1...Qc5 2.Qd4#
1...Gg7 2.Bf2#

In the first three thematic variations Black plays onto a line between a wG and the bK and White can then mate with his queen on the square attacked by that black piece and next to the bK on that same G line. If the black piece were to capture the Q, it would be self-check from the G. So the black piece is pinned by the mating piece. Such indirect pinning of a black piece on a mating move, is, I found out just recently, called the Boros theme. These days I would use rook-hoppers and bishop-hoppers instead of Grasshoppers where I could.