Mate in 6

The main idea is 1.Bxg6, aiming for 2.Rf5#, but 1…b1=Q (or B) throws a spanner in the works. To meet this defence, White must first carry out a subtle foreplan:

1.Na5! threat: 2.Nc4#

1...Kd6 2.Rf7! threat: 3.Nc4#

2...c4 3.Nb7+ Ke5

This sly manoeuvring brings about a crucial change: the bP that was on c5 has moved to c4, a small yet pivotal shift that now enables White to realise their original plan.

4.Bxg6! threat: 5.Rf5#

4...b1=Q/B 5.d4+! cxd3 (e.p) 6.Rf5#

White forces an en passant capture, shutting off the b1–f5 diagonal — exactly the point of nudging the bPc5 a square forward. Speak of cunning!