When the enemy king’s position is well-fortified a sacrifice may be needed to loosen it up. Perhaps the most common sort of fortification is a rook next to the king with cover in front of it, typically as a result of castling. A classic way to remove such a rook is by sacrificing a heavy piece on the square in front of it, pulling the rook up off the back rank. There are various ways to then finish the sequence. The first, which we consider now, involves then throwing two coordinated rooks at the enemy king’s position.
In this first example Black’s king seems well-protected; his rook on d8 protects the back rank. But White has a battery directed down the d-file, suggesting that the d8 rook might be drawn forward and out of position. First comes a sacrifice on the square in front of it: 1. QxB+, RxQ (to prevent White from playing QxR#). Now 2. Re8+ forces Black's rook back to d8—and now it is pinned to its king with rooks aimed at it from two directions. White mates by playing RxR with either of his rooks.