Jörg Kuhlmann & BDS
The Problemist Supplement, 1998
#3
Sentinels
1.Sf6+? Bxf6[+bPg7]! 1.Bb3! (2.Be6[+wPb3] & 3.Bg4[+wPe6]#) 1...Bh8[+bPg7] 2.Sf6+ gxf6 3.Rh7[+wPb7]#
White wants to mate by playing his rook to h7, but 1.Sf6+ Bxf6! leaves a black pawn behind at g7, so access to h7 is blocked and after 2.Rxg7? Bxg5! defends. Black's only defence to the quiet threat carried by the key is to stalemate himself by blocking his bishop in at h8, but after that 2.Sf6+! does work because now it is a black pawn that captures and that doesn't leave another pawn behind, so clearing the way to h7.
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