This is a total n00b question but it's something I haven't understood despite watching football for so long now:
Obviously, I understand the penalty for defense having 11+ players on the field AT THE SNAP. What I don't get it is when well in advance of the snap, there is a penalty for defense having too many on the field? In what circumstances is this illegal such that there is a penalty BEFORE THE SNAP? Broncos seem to have had this called against us a few times this season, and I don't think I get when the penalty is called at the snap versus whistling the play dead well in advance.