Nine in the second place is the most reliable reading in Great Power and also the least dramatic. Firm correctness brings good fortune, and that is the whole of what the text offers. A querent landing here has strength available and the sense to keep it where it belongs, and the commentary's appeal to the centre is the practical instruction: act from the middle of your own position rather than from its edges.
Structure carries all of it. This is the strong central stroke of the lower trigram Qian, answered by the yielding fifth line in the trigram of thunder above. By the ordinary rule a firm line in an even seat is incorrectly placed, so what this line holds is centrality rather than perfect propriety, and in a figure about force the classic treats the first as worth more than the second. Being in the middle is what keeps power from turning into display.
When the line changes, the lower trigram Qian becomes Li and the figure becomes Abundance, King Wen 55, greatness at its fullest with the warning that the sun at noon begins to decline. That is the natural consequence of well governed strength and the caution that travels with it. Power kept central accumulates, and what accumulates eventually reaches a height from which the only remaining movement is downward, which is a difficulty this line does not yet have.