Trace the lines leading out from Black’s king and see that the rook on c5 is positioned to be pinned. Consider whether White has any pieces with the power to impose the pin; he does, in the dark-squared bishop on e1. So you turn to the obstacles to carrying out the idea, and find that the bishop can’t get to the pinned square—b4—because its fellow rook is in the way. White’s job is to clear the rook from c3, and to do it violently enough to allow Black no time to avoid the pin that follows. The natural way to do it is with a check or capture. Here the capture works: 1. RxR, RxR, and now Bb4 pins and wins the rook.