Moving the king <BD>b2-d4</BD> accomplishes two separate goals: first, it promotes the king toward the center; and it also establishes a path that supports a jump: <BD>a2-e4</BD>.

Move: <BD>b2-d4</BD> and observe black's response (<BD>h7-f5</BD> establishes a path for black's king: <BD>g7-e5</BD>).