Royal Piece Se2 =============================== 1... ... 2.Sf7 f5 3.Bf4 rSxf4 [brK=rB] # - not 4.rBxf3(wrS=wK)?? - self-check!! 1...rSd4 2.Sg4 rSf5 3.Bf6 fxg4 [brK=rS] # - not 4.rSxf4(wrS=wK)?? - self-check!! The black King in the rB/rS phases can't capture the white pawns f3/f4 because it transforms white rS into the King phase giving the self-check. Sacrifices of the black S/B with the transformation of the black King.