Setting the chains

[Jim D]

You have to enforce the penalty first and then see where that enforcement puts the ball to see if they end up with a first down. For example, if a player rips off a 30 yard run but the offense was holding at the line of scrimmage, you have to assume he was able to gain the 30 yards because of the hold and, therefore, it would be unfair to give the offense a first down on this play and then mark off the penalty yardage.