What White pieces are loose? The bishop at c3. Black needs a way to attack it and attack something else at the same time—preferably the White king. His queen is off the board, so what might he do with his rook? He can attack the loose bishop with Rd3. That move doesn’t attack the king directly, but the opportunity presented by the loose piece is important, so think harder; examine the White king’s position carefully. The bishop at b6 cuts off the g1 square. The bishop on e4 pins the pawn on g2. So if Black could get a rook onto the h-file, it would be mate. This idea can be put together with the previous one: Rd3 both attacks the bishop and threatens to end the game with Rh3#; after White fends off the mate threat, Black plays RxB. The lesson is to always be aware of the enemy king (not to mention your own) and any of your pieces that constrain it.
The motif illustrated by this position—the fork that targets an enemy piece at one end and threatens mate at the other—is familiar from the previous chapters. It occurs less often with the rook than with the other pieces we have considered because the rook less often is in position to do both of those things. We consider it here only briefly.