I wonder what the exact rule is, Vols were up 48-0 at half and I don't think we employed any such rule

Edit: it seems that both coaches have to agree to it, but it can happen at any point, see below:

• The National Collegiate Athletic Association's mercy rule provides, "Any time during the game, the playing time of any remaining period or periods and the intermission between halves may be shortened by mutual agreement of the opposing head coaches and the referee." (NCAA Football Rule 3-2-2-a)