A scan of the Black king’s lines again turns up a pin already in place: of Black’s rook, by White’s queen. But the pin is imperfect because the rook is guarded once and attacked once. White has no other pieces he can use to put immediate pressure on the rook; Bf4 fails to QxB. When you seem to be out of resources, however, consider exchanges and their consequences. Here QxR leads to QxQ—and now White imposes a fresh pin with Bd4, as the bishop gets protection against the pinned Black queen from the pawn on c3. Be mindful of the possibility this position illustrates: the ability of the pinning piece to initiate useful exchanges.