The Rook on Q5 is immobile because pinned by the White Bishop. White attacks it by X-K2. Now it is twice attacked and guarded only once. Black keeps the balance by R-R5. Now White plays XxR, so as to immobilise the other Rook. Black may reply RxX, but only to lose that rook, since White assails it anew by K-K3, thus gaining the superiority of two against one upon the disputed point Q5.
The idea of superiority is of frequent use. I intend, for instance, to put a Pawn or a piece upon a point of vantage that my opponent will not let me gain and to obtain my purpose I must support that Pawn or that piece upon the point aimed at by units in number superior or at least equal to those of the defender.