I took this from the "Jacob's cabin" review right here on Lostpedia, coming from the Trivia section,
"According to commentary for "The Man Behind the Curtain" on the Season 3 DVD by producers Carlton Cuse and Damon Lindelof, the circle of ash is "a bit of a kind of protection or magic or kind of containment. Ben is afraid to touch it. In a certain way, part of the idea here is that Ben has some powers over Jacob, and Jacob has some powers over Ben, and there is a bit of a stasis."
I don't know about you guys, but that smells like a load of BS now. Stuff like that really makes me feel they had little knowledge on where they were going with things. I mean, what the hell are the producers even trying to say here? That Ben was keeping Jacob prisoner? That the old scary grey haired dude Locke saw was Jacob? He said "help me"!?
Please, someone enlighten me