Mate in 6

Clearly, White must leverage their dark-squared bishop to create threats along the f8-a3 diagonal. A logical attempt is 1.Bg7?, menacing 2.Bf8#. However, 1...Rf2! effectively defuses this threat, and White's mating prospects evaporate as they must now address the reciprocal threat of ...Rf1#. Therefore, some groundwork is required to make Bg7 work.

1.Be5! threat: 2.Bd6#

1…Bg3

2.Nf4! Bxf4 (2…gxf4? 3.Bd6#)

3.Bg7! threat: 4.Bf8#

With the f-file closed, Rf2 is now of no avail. How does then Black reckon with the threat of 4.Bf8#? 3...Rc2 runs into 4.Bf8+ Bd6 5.Bxd6+ Rc5 6.Bxc5#. Similarly, 3...Re2 is met with 4.Bf8+ Re7 5.Bxe7+ Bd6 6.Bxd6#. Hence, the only reasonable defence seems to be:

3…Bd6!?

But in a surprising twist, the occupation of d6 also proves disastrous.

4.Ra6+ Kb4

5.Ra4+ Kc5

6.Bd4# Model mate.

This exemplifies the remote self-block effect, a subtle tactical motif where a piece's self-block on a distant square becomes decisive when the king moves into proximity.