Help-Selfmate in 2 (2 Solutions)

With the rook on b8 pinning Re8 and the queen on h7 covering key flight squares of the black king, their roles are already spoken for — leaving the burden of action to the remaining white trio: the rook on b2, the knight on d2, and the bishop on e1. bRf2 and bBb4 are the likeliest candidates to deliver mate — the former along the fourth rank, the latter along g1–a7 — hinting at a possible orthogonal-diagonal relationship between the two solutions.

If the knight on d2 were plucked off the board on move one, two sequences leap out — one ending in a rook mate after RxR and BxB, the other in a bishop mate following BxB and RxR. As it turns out, each can be realised by a different knight move: 1.Nf3 Rxb2 2.Bxb4+ Rxb4# and 1.Nb3 Bxe1 2.Rxf2+ Bxf2#.

A crisp instance of the Orthogonal–Diagonal Transformation (ODT), and Zilahi to boot: A unit captured in one phase (bR/bB) returns to deliver mate in the other — and vice versa.