Here Black’s king still is settled on the back rank, so at first the possibility of a skewer may not seem obvious. But consider any checks White can deliver. He can move either rook to e1, which allows Black to interpose his own rook at e7 or move his king along the back rank. More interestingly, White can play the rook on h1 to h8. This forces Black to move his king to the seventh rank—and into line with the rook on c7. Now White moves the rook from h8 to h7, and the king has to move again. Notice that it cannot protect the rook it will be exposing to capture; White’s rook on d1 bars the king from the d-file. White wins a rook.