So if Team B fumbles the ball, they are then responsible for where it goes. They are responsible for it until there is a new impetus. As long as the ball is bouncing around, they are responsible for it going into their own end zone.
So, in the scenario I offered (B catches A's kick in B's end zone, fumbles ball directly OB behind GL), you would have a ruling of a safety. After all, the impetus in now with B's fumble, and it is dead behind B's goal line. That is the definition of a safety, by 8-5 and 8-7, and by the Play Situations Bulletin Redding issued in 2001, and with his banishment of AR 8-7-2-III, which was the ONLY thing that said otherwise.
Like I said, if you believe impetus is what put the ball FROM THE FIELD OF PLAY into an end zone, then touchback would (and SHOULD) be the ruling. Unfortunately, the rules have never been written that way, although that was how Nelson and Adams interpreted them. When Redding got rid of AR 8-7-2-III and issued the 2011 bulletin, things are now very...different, to say the least.
Quoting Rom, "Impetus is charged to the team that is responsible for the ball going
from the field of play into B's end zone." But there is NO rule or AR support for that statement, as much as I wish there were.
Here is the 2011 Play Situation:
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Fumble in End Zone Following Interception
2. Third and five at the B-20. Defensive back B44 intercepts a forward pass at the B-3 and his momentum carries him into his end zone. While still in the end zone he fumbles the ball. It rolls forward, goes into the field of play, and in the scramble the ball goes back into the end zone (a) where B44 recovers. He is tackled in the end zone; (b) and over the end line.
RULING: Safety, two points for Team A, in both (a) and (b). The impetus for the ball The scramble to recover the ball does not add new impetus. The momentum rule does not apply because the ball did not remain in the end zone. (8-5-1, 8-7)
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So, if you make the case that, because the ball went from the end zone into the field of play from B's fumble, and then crossed back into the end zone before going OB, that should be a safety, then you can't say that impetus is what puts the ball from the field of play into the end zone. By that, impetus applies regardless of where inbounds it occurs. So, when B gains possession in the end zone, then fumbles the ball directly OB in the end zone, that should be a safety. For over 45 years, that was not the case, and I don't think it is the case, currently. But, there is no book support for a touchback.
If you subscribe to the concept of impetus being the force that puts the ball from the field of play into an end zone, and that specific impetus remains with the ball until changed by some other impetus, regardless whether or not the ball happens to bounce/roll back and forth across the goal line (something which I know for a fact that Adams said made no difference) then the ruling in Rom's play situation would be to apply the momentum rule (not a safety). There. I finally got to how this whole discussion might apply to momentum. But, Redding says safety. OK, if that's the case, what about my scenario? Safety? Gosh, I hope not. Not when it was a TB for over 45 years.