Figure 5.3.9.4[White to move]

Black's king can’t move to d8, and f7 is off limits as well; indeed, it has no flight squares—and there is an open diagonal leading toward it. We know a way to take advantage of this: try to get an attacker onto the diagonal, exploiting the king’s cramped position. White looks to his queen and light-squared bishop, and finds the move Bf7—almost mate, with protection from the knight on e5, but f7 is guarded by Black’s rook on h7. When an otherwise effective mate threat is impeded by an enemy piece, we know how to think about this; we make the troublesome piece a focus of operations in its own right. Capturing the rook is not possible, but White can get an attacker onto the h-file with Qxh5+. Notice that in addition to attacking Black’s rook, this move gives check; this has the usual importance because it makes Black’s choice of replies much more limited than it was in the cases where we tried plainer attacks against enemy pieces. Black has no way to protect his king except RxQ (or Rf7); and White mates on f7 with his bishop.

Lesson: if a troublesome enemy piece can be attacked with check, this generally is an attractive way to go; the threat to the enemy king simplifies your analysis of the consequences.