There's an excellent article posted by Scott Ehret which explains the entire concept. Scott, as you may recall, has been a rules interpreter for Referee Magazine and NASO for many years . . . and is highly respected in the area of "common sense" officiating.
http://www.amateurumpire.com/mech/mech08.htm
Warren . . . I'm surprised you weren't familiar with this article. It was written during the time when Papa was involved with NASO.