AG has an explanation for why he does the valve drill by shutting the left post and breathing the reg down . . . it has to do with cementing the association between the post and the reg. I didn't find the argument convincing, so I didn't remember it very well. GUE does the sequence the way they do because it more closely approximates what you would do in the event of an actual leak -- when you wouldn't be breathing the backup reg, so you wouldn't breathe it down, but purge it instead. These are really minor, picky details in my view.