White sees the tempting pattern of queen in front of king on the e-file. He instinctively thinks of the classic rook pin Re1. But on consideration of the likely reply he sees that Black has an interposition to play: Ne4, blocking White from playing RxQ. Is White dissuaded? No, because a piece that defends by interposing itself often becomes pinned itself, whether to the king or queen—as happens here. The knight would be paralyzed upon its arrival at e4, at least momentarily, because if it moves it exposes the queen to attack. So White goes after the knight with a pawn: f2-f3. He has f3xN next move.