If you are referring to a rule there. I don't have a rule book. I am not an official. I am a simple fan trying to gain an understanding of the rules.
Can you dumb it down?
There were fouls by both teams on the play. Normally, those would offset and the down would be repeated. Had the fouls offset in this case, BYU would have kept the ball, and the down would have been repeated. However, there is an exception to the 'offsetting' rule that allows the team in possession at the end of the down to keep the ball, but only if they did not commit a foul until after they gained possession of the ball. And that is what happened. BSU committed a block in the back during the interception return. This is known in officiating as the "clean hands" exception, because the team in possession at the end of the down were free from foul - their hands were clean - when they gained possession. It would be a bit unfair to have to give the ball back to their opponent when they made a good play before they fouled. So, they can decline their opponent's penalty, and their opponent can then accept or decline their penalty - almost always accepted.
However, they could have allowed the fouls to offset, and repeated the down. Something very unusual - nearly rare.
If there are fouls by both teams, and the team in possession at the end of the down fouled before last gaining possession, then, by rule, those fouls offset - no choice.
