White couldn't move a B in c8 or h3, because these moves cause a check, and not even in a6, since this square is occupied by a white pawn. B1 allows one of the B moves. In two solutions the bQ forms an anti-battery with a RL and double check; in the third solution a bRL executes a critical move and sacrifices itself capturing the wP.
In B2 the bK indirectly unpins both wS and simultaneously it upins a black unit. In two solutions, the Sg7 moves in W2, exploiting the potential check line destruction, and it guards two squares; the Sb2 moves instead in the third solution. The unpinned black unit moves in B3 to open the line to the second wB and to block a square. Finally, model mate by the other wS, with role reversal of the two wS. (Author)