 To the left is one of Rusinek's better known studies.Black threatens Nb5-d6# or Ne4-d6# and 1.Kb7 Bd5+ does not help, so 1.a7 is necessary.Now 1...Bd5 can be answered with, among other lines, 2.g8Q Bxg8 3.a8Q Nxb6+ 4.Kb7 Nxa8 5.Kxa8 Be6 6.Kb8 and Black must give up a piece for the c pawn, so instead 1...Ba6+ 2.b7.