Nine in the third place asks a querent to hold something back, and it says what happens when they do not. The ram charges the fence and its horns are caught, so the strength that broke the ground is now the reason nothing moves. The text calls persistence dangerous even where the person is in the right, which is the specific case this line addresses: not power misused, but power fully used by somebody entitled to use it.
The distinction the classic draws is between a small man who spends all of his strength and a superior man who keeps a rule against doing so. That is the period's own way of sorting people and it stands here as the text has it. Underneath the vocabulary the counsel is exact and portable. A reserve is not kept out of timidity; it is the thing that lets a person disengage, and the ram in the fence has nothing left to pull with because everything went into the charge.
When the line changes, the lower trigram Qian becomes Dui and the figure becomes The Marrying Maiden, King Wen 54, an entry into a relationship on terms the one entering does not set. That is the shape of being stuck, read forward. Horns caught in a fence describe a position from which the next arrangement gets negotiated from weakness, and the line is asking the querent to notice that before the charge rather than afterwards.