The third line performs the second line's choice in reverse and gains something by it. Legge's translation shows its subject cleaving to the man of age and experience and letting go the little boy, states that such following will get what it seeks, and then attaches a condition, that it will be advantageous to adhere to what is firm and correct. The gain is granted immediately; the warning arrives with it, and the two belong together.
A yielding stroke stands in an odd place here, incorrectly positioned by the rule, at the top of the lower trigram Zhen and reaching up toward the strong fourth line above. The reach is what the line is. It gives up the nearer attachment below in favour of the more substantial one further off, which is precisely the trade the second line declined to make, and its incorrect placement is a fair picture of the discomfort involved in stretching past what is comfortable.
Why the condition is needed becomes clear once the gain is taken seriously. A person who has attached themselves to something worth following gets what they were after, and getting what one was after is the point at which the following can quietly turn into leverage. Firmness here means holding to the relationship on the same terms it was entered on, so that the pursuit does not become a claim on the person pursued once the association has started paying.