When on the waitlist and your requested villa becomes available, a Member Services cast member will call you to verify that you still want to make the reservation or change. They'll only hold the room until the close of business on the day after they initially attempt to contact you.
If you've requested automatic confirmation, though, they'll simply make the reservation instead of attempting to contact you first. Then, they'll send you a confirmation. Note: automatic confirmation is only if the waitlist request becomes available 60 days or more before your check in date. If your check in is closer than that, they'll call. If your request isn't filled by seven days before your check-in, they'll take you off the waitlist.
Also keep in mind that waitlist requests only remain active for a month. If you want to stay on the waitlist longer than this, you have to call Member Services and ask them to extend your waitlist request.