When setting up a registration within your event and on the Event Registration tab, you have the option to allow for a waiting list by checking the following setting (see below).
So when your registration shows as full, registrants can still register to the waiting list.
If a registrant is removed from the registration, all on list are notified by email simultaneously every time a ticket becomes available. This is what the email looks like (see below).
The registrant on the waiting list can then click "Get Ticket" to register for the event.