The moment the page turns from shopfront to signpost
A registration page spends most of its life as a shopfront: promise, agenda, speaker, button. In the hour the seats run out it becomes something else โ a signpost. Someone clicks through from a friend's forward, reads that the room is full, and makes a decision in about ten seconds about whether to care.
With the waitlist switched on, caring has somewhere to go. The visitor leaves their details the same way a registrant would, and the page tells them plainly that a spot opening means an email. Without it, the full notice is a wall, and the person who was genuinely interested just closes the tab and forgets your name by Thursday.