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Comm., The Problemist, 1997

#2
Patrol Chess

1.Sc6! (2.b8=S#)

1...Qg5 2.Sfe7#
1...Qd2 2.Sfd4#

After the key the black king is not in check (because the white knight c6 is not covered by another white piece), but the threat does cover that piece and so the black king would be mated after the threat. Note that neither 2.Sfe7 nor 2.Sfd4 is a threat because that knight is pinned, the white bishop b1 being covered by the bQ.

Black defends by guarding his knight on d8 with his queen so that, if the threat were played, the knight could capture the checking knight on c6. In moving his queen though, Black is no longer covering his bishop, so this effectively unpins the white knight f5, which can then mate by guarding the other white knight. The choice of mating square is dictated by the need to interrupt the black queen's guard of the black knight on d8.