I read it as being related to the "line in the sand" idiom. There exists some set of rules, the "line". Exactly what is and isn't allowed under those rules is a bit arbitrary, like the exact location where you would draw a line in the sand with your finger. What matters is that the line has been drawn, and everybody knows that the line may not be crossed.

Under that metaphor, a person may stay very far from the line, to avoid accidentally stepping over it, or they may walk right up to the line. Metaphorically, the former would be a person who refuses to answer any questions about nuclear secrets, regardless of whether the question can be legally answered. The latter would be a person who knows exactly what can be legally answered, and will give as full of an answer as is allowed. They know where the line in the sand is, and have walked up to the line.