Mate in 2

White has a mate lined up for each of Black's potential moves, placing Black in zugzwang. If White could simply pass their turn, checkmate would follow next:

1...Rg1 or any other move of the g2 rook opens up the h1-a8 diagonal, resulting in 2.bxa8=N#.

1...Kxb6 is met with 2.bxa8=N#.

Lastly, 1...Na6+ or any move of the b8 knight runs into 2.b8=Q#.

The problem is tricky because White lacks a spare move to maintain these set mates. So what are we to do? The key move is 1.Rb5! – this also induces zugzwang, but alters the mates for some of Black's responses.

1…Rg1 or any move of the g2 rook is now answered by 2.bxa8=Q# (instead of 2.bxa8=N). And 1…Nc6+ or 1…Nd7+ by 2.b8=N# (in contrast to 2.b8=Q of the set play).

The move 1…Na6+ still elicits 2.b8=Q#, while 1...Ka6 falls to 2.bxa8=Q#.

This is a fine example of what we call a mutate in chess problem terminology. A mutate is a type of block problem – one where the key sets up a zugzwang without a threat – in which at least one mate in the set play is changed in the actual solution.