Complex sentence: If the offense doesn't get the ball hiked before the 20 seconds is down, then they will get a "Delay of Game Penalty" but the referee will tell the offense when 10 seconds is left.
Split into simpler sentences: If the offense doesn't get the ball hiked before the 20 seconds, then they will get a Delay of Game Penalty. But the referee will tell the offense when 10 seconds is left.