Here is the verbatim quote of the "Yellow Line Rule;"
"This is your warning: race above the yellow line. If, in NASCAR's judgment, you go below the yellow line to improve your position, you will be black- flagged. If in NASCAR's judgment you force someone below the yellow line (in an effort to stop him from passing you), you may be black-flagged."
For driving below the line, you WILL be black-flagged, for forcing them down there, you MAY be black-flagged.
There is the fine print.
But don't forget that Mike Helton said this in a meeting in 2003;
"What we are beginning to see happen, and we saw it [in the Busch race], is some of you have figured out that to protect your spot, you can move down against the yellow line to prevent someone from going on your inside," Helton told the drivers. "That is OK. But if you do it while that guy is trying to get around you and you move him down there while he has a quarter-panel or fender alongside of you, if you make him go down there, then you are subject to a black flag, too."
At about 40 seconds in, Smith begins to move outside, then back inside, and Stewart CLEARLY went to block outside, then inside, and Smith very much had his right front inside his rear quarter panel and was forced below the line. I don't think Smith could be spared, but Stewart may have deserved the "pass through" penalty as well, as per the rules, and Mike Helton's statements.
Personally, in hind-sight, I would have either just let him get spun out, or I would have juked inside, then out.
But that's me.