My first instinct was Nd2 because you release the attack on the a1-h8 diagonal, hit the king, force him to walk into d4, pinning the knight and then… IF there was another pawn on f2, blocking the escape square of e3, Bxf6 delivers mate in 2…so that’s my guess of what it’s supposed to look like. Or any other piece that takes away e3 but I think you’d be limited to either rooks or queens and since they could deliver the mate themselves it would introduce alternative solutions so not as desirable as the pawn…
My first instinct was Nd2 because you release the attack on the a1-h8 diagonal, hit the king, force him to walk into d4, pinning the knight and then… IF there was another pawn on f2, blocking the escape square of e3, Bxf6 delivers mate in 2…so that’s my guess of what it’s supposed to look like. Or any other piece that takes away e3 but I think you’d be limited to either rooks or queens and since they could deliver the mate themselves it would introduce alternative solutions so not as desirable as the pawn…