You can enable a waiting list for any of your ticket types. It is all set up to work automatically to make sure you sell as many tickets as possible.
How to enable it
You need to enable it for each ticket type, so you are able to have waitlists for some ticket types and not for others. Check this article on how to set the options.
Next you need to set some global ticket settings regarding how many hours people on a waiting list have to react and register for a newly available ticket. That's done under ticket settings.
How it works for attendees
When a ticket type is sold out, the option to buy the ticket is replaced by an option to sign up for a waiting list (for the specific ticket type).
When a ticket becomes available, then the first person to sign up to the waiting list receives an email with the option of buying it. S/he then has a 12 hour (default, but you can set it to be shorter/longer under ticket settings) reservation time to acquire the ticket. When the reservation time expires, an email is sent to the next on the list with the offer of purchasing this ticket.
How does it work for me?
You can see how many people and who is on a waiting list, once the first person has signed up for a spot on the waiting list.
Notice that a place on a waiting list is tied to a user account, so you can see exactly who is on a waiting list for your event.
How do I release more tickets for a waiting list?
A waiting list spot is released under these circumstances:
If you increase the amount of available tickets (i.e. you add 10 more tickets), then the first 10 people on the waiting list for this ticket type receive an email with a reservation and purchasing offer).
If a ticket is cancelled by the ticket owner or by yourself.