Figure[White to move]

Here is a somewhat less usual configuration that depends on a similar principle. Where does White have two pieces aimed at a square next to the enemy king? On the long diagonal, where he has a queen and bishop (the rook on g2 bears down as well, but for now focus on the other two pieces). The battery seems imperfect because the bishop is in front, but still: notice that none of the pieces around Black’s king can defend the long diagonal. The only piece that can reach it in principle is the knight on f7, but of course you see that it is pinned by White’s other bishop on e6. The obvious obstacle to penetrating the Black king’s position is the Black pawn on e5, so play through the sequence that would be possible if the pawn were gone: 1. Bh8; and on reflection there would be nothing Black could do to stop Qg7#. This is the significance of Black's inability to defend the diagonal.

What does this mean? It means the pawn on e5 is pinned, since if it moves off the long diagonal White finishes Black’s king. Rather than obsess over the difficulty of removing the pawn, exploit its immobility by taking something it protects—with RxN.