Our current logic continues. Everything starts by spotting the kernel of the discovery. Here White’s knight on h4 masks his rook. (When the h-file is open and the enemy king has castled on that side, be alert to the relationships between any pieces you have on that line or able to reach it.) A good first question is whether the piece to be unmasked has a promising target—and, if not, whether one can be created. In this case there's nothing for the rook to take at the other end of the h-file, but the king is close by; might it be moved over? A natural way to accomplish this is with a check, and especially by a check from a piece moved onto the h-file that the king will be obliged to capture there. So consider Qh7, which requires Black to play KxQ. It’s an expensive sacrifice, but since the enemy king is in play it might be worthwhile. Now Ng6+ unmasks check by the rook; what would Black do then? Kg8. Look for a final check and you come to Rh8#, with the rook receiving cover from the knight.