Discuss this rule with a mandolin teacher or guitar player if you have one you can consult. This music has a particular pattern that gives the jig the appropriate bounce. In general, the exceptions deal with the fact that on occasion it's necessary to change direction to emphasize the particular sound you're searching for. Typically those are fairly advanced circumstances. For now, use this information if you choose, to evaluate weakness in passages that may be problematic to you.
Chances are they involve the proper pick direction.