"'Who started it?' asked Atticus, in resignation.
'Jem did. He was tryin' to tell me what to do. I don't have to mind him now, do I?'
Atticus smiled. 'Let's leave it at this: you mind Jem whenever he can make you. Fair enough?'" (138).
This new declaration of policy between Jem and Scout helps to even the playing field in a way. Jem is older and more mature, but Atticus knows that it is difficult to obey a sibling. In this case, Jem had taken Scout aside to reason with her about not ruffling Aunt Alexandra's feathers. He may have persuaded Scout to mind him in this endeavor had he not said that he would spank her the next time she behaved that way. Jem will have to "make" Scout mind him in clever ways, not physical ones. If, however, Jem does resort to using physical means to "make" Scout mind him, he knows that she will fight back--and there was at least one time she was able to land a strong punch on him. Be that as it may, Jem is still older and stronger. If he became truly upset, he could hurt Scout pretty badly if he ever resorted to physical violence to make her mind.
Fortunately, Atticus must trust Jem not to physically hurt Scout, even if he does threaten it. Atticus must also know that tempers flare, but his kids tend to get over it pretty quickly, too. In the end, this ambiguous rule leaves the sibling relationship in the hands of the brother and sister who must now work together more effectively if they are to get along. Fortunately, Jem never takes advantage of his physical advantage over his sister and this rule never really has to be addressed again.
What the rule means is that Scout, who is the little sister, has to mind (obey) Jem whenever Jem can force her to obey him (presumably by physical force.
To me, the clear problem with this is that it is going to lead to a lot of fights. Scout is going to say "you can't make me" and Jem is going to try to make her. That's just what is happening in Chapter 14 when Atticus makes this rule. The other problem is that it implies that whoever is strongest gets to make the rules and that does not seem right (it also seems unlike Atticus to suggest this).
In any other family besides the Finches, this type of "rule" by the father might result in physical force. However, I think this is a genius rule that will not in fact lead to physical fights. Atticus knows this too, otherwise he wouldn't have mandated it.
Atticus knows that the one thing both his children want the most, is his respect and attention. Atticus has never spanked either of them at the ages the book takes place (at least to our knowledge), yet, both Scout and Jem do their best to obey Atticus. In giving this rule, Atticus is sending two very clear messages: one - that because Jem is older, he is worthy of a certain measure of respect himself, and as a big brother, he is capable of giving big-brother advice (which Atticus realizes is what Jem is ultimately trying to do but isn't yet comfortable in this role), but two - Scout, you never have to take this advice if you don't want to.
If Jem learns how to make Scout mind him, what he will have mastered is his relationship with her. And THIS - is Atticus' key parenting strategy. Like I said, genius.
The obvious problem this might cause is that Jem could interpret the word make differently than Atticus or Scout would want him to. In most sibling relationships, the older dominates the younger and often that is done (no matter how much parents try to avoid it) through physical and mental dominance. Thus, this gives Jem the green light to go ahead and inflict pain on Scout to get what he wants as long as Atticus and Calpurnia aren't there to defend Scout.
On the other hand, an older sibling often does have a stronger grasp of behavior and can help parents encourage a younger sibling to act appropriately, particularly in situations when parents are missing.
Bummer for Scout.