UCF vs Iowa St. game and on a lot of those big run plays, the Iowa State's dbs are locked in man to man, which meant their backs are turned to the play. By the time time the db realized the qb or rb was downfield, they were in no position to make a td saving tackle.

That's why it seems to me that if they aren't going to be passing a lot, you may fake man cover, but drop into a zone so you have as many eyes in the backfield as possible. 

On passing downs, man cover with a spy to keep the qb from picking up 1st downs with his feet.