Black's king threatens White’s rook. What guards the rook? White’s king. Can Black check White’s king and thus drive it away? Yes, with Rh4+. White has to move his king back to the third rank, leaving his rook loose and thus permitting Black to play KxR. But wait: what if White instead replies to Rh4+ by interposing with Rg4? Then the rook is attacked twice and protected once, and is lost—but only because of the bishop on e6, which thus is essential to make this sequence work.