Here we see that Black’s bishop already is pinned, but it seems safe because it's attacked twice and protected twice. White could add to the pressure with e4-e5, but then Black has BxB; indeed, he is poised to play that move next in any event. So White’s natural idea is to force a substitution. Thus 1. RxB leads to 1. …RxR (KxR is out for Black because the square remains under attack; his king can’t move there). The pinned Black piece has been changed from a bishop that can defend itself to a rook that is genuinely paralyzed and that can’t lash out against the bishop that pins it. Now e4-e5 works, as White takes the rook with his pawn a move later no matter what Black does.