We turn to the power of the rook to discover check. The position on the left illustrates the pattern in simple form. Scan the diagonals and you see Black’s queen behind his rook, with White's king at the other end of the line. The stage is set for a discovered check; Black just needs a good target for his rook. Best would be either White’s queen (because it’s more valuable than the rook) or any unprotected White piece (because then it doesn’t matter whether the rook is more valuable). White’s queen can be attacked with Rc4+; after White protects his king (Kh1 is best), Black plays RxQ. It would be different if White could take his queen someplace where it had protection and blocked the check, but there is no such square. If White plays Qe3, for example, Black has QxQ+; this renews the check and thus gives Black time to save his rook on his next move.