TxMike is correct. This year, if any part of the back is inside the tackle box, modified or otherwise, he is unrestricted. He is restricted when completely outside the tackle box.
We impressed the point to the guys at our clinic that this is a change from last year. In our conference, the wing guys will have to make that determination, and the guidance is "when in doubt, he's out" relative to the tackle box, and therefore restricted.