It is a rule around substitution. When the offense subs in people the rule is the defense gets an opportunity to sub in people as well. The ref won't allow the ball to be snapped until the subs are on the field and ready to go. If the offense didn't sub people in then the ref wouldn't hold the snap.
Comment 01 Jan 2015
Comment 16 Jan 2013
I am not sure who this story says more about: Deadspin for doing there homework or the rest of the media for not verifying things.