NCAA rule - dead ball on first down

[Dakota]

Has the ending of the SC - Auburn game been discussed here? If so, the thread title isn't obvious. If not, I'm surprised.

I have no fan or other affiliation with either team, but I am curious about how the rule is administered when a first down is achieved. (I have only a fan's knowledge of football rules...)

It seems to me the dead ball can be administered either immediately upon the player being down or upon the official's signal.

Of course, if SC had done a better job of clock management, it would not have come down to that, but I was wondering... how is the rule actually administered?