Black has just played his bishop to c5, where it checks White’s king. White considers his options: capture the bishop with his queen (out of the question since the bishop has protection); move the king to h1 (plausible); or interpose something between his king and Black’s bishop. What interpositions does he have? Bd4 would seem to work; the bishop would have protection from the queen on c4. Any problem with this? Yes; when you interpose a piece to block a check, remember that this causes the interposing piece to become pinned. Any defensive purpose it had been serving is no longer being served; any pieces it was protecting may no longer be protected. So before interposing the bishop White must ask whether bishop is supposed to be protecting anything. It is: the knight on e5, which will be lost to QxN if the bishop is paralyzed. So Bd4 is the wrong move for White; he should move his king.