Thats incorrect I believe. I've always understood that Standbys are required to be there the entire show, always ready to go on. The night at Wicked you are referring to was a night on which LBB had a planned absence already, otherwise she would have gone on. It wouldn't make sense for the understudy to be rushed on, because then they'd have to also pull a swing into the understudy's role. Standbys have to be there every night, just like anyone else, as they are the first in line if something happens to the lead.
Or at least, thats how I understand it.
~m~
For heaven's sake, don't try to be cynical. It's perfectly easy to be cynical.
~The Importance of Being Earnest~