Figure[Black to move]

This time Black’s b7 pawn masks the horizontal path of his queen. This idea would most easily be triggered by the sight of White’s bishop loose on a7; when the enemy has a loose piece, you think hard about whether any of your pieces can find a way to attack it—including any pieces aimed at it whose paths might be cleared somehow. Black almost is ready to go with b7-b5, attacking White’s rook and unveiling the threat of QxB. But consider White’s replies and whether he would be able to go on the offensive himself, especially since Black’s move wouldn’t give check. White would indeed be able to play RxR+. Black then would have to play QxR, ending the threat to White’s bishop with no gain.

So again a preliminary exchange is in order to upgrade the target: Black starts with RxR, and after White recaptures with QxR the calculus runs differently. White has no good answer to b7-b5. He has to move his queen and endure QxB by Black next move. (White would like to move his queen someplace where it safely can defend the bishop, but it has no such square.)