Figure[Black to move]

Breaking a Skewer.

White has just skewered Black’s king and rook. Must the Black rook be lost? Among other things Black is confronted with a check, so in principle he can deal with it in one of three ways: capturing the menacing piece, moving the king, or—the only thing that helps here—interposing something. Bd8 blocks the skewer and (importantly) gives the bishop protection from the king behind it.