A scan of the Black king’s lines turns up an obvious candidate for a pin in the knight on e4. White plays the simple 1. Re1. Now what? Black rushes a pawn to the knight’s defense with d7-d5, which rules out RxN for White on the next move. So White resorts to a pawn: 2. f2-f3, winning the knight a move later. The protection a pinned piece enjoys becomes meaningless when it is attacked by a pawn.