If you are admitted off the waitlist, you will receive a phone call, which is basically along the lines of "are you 100% certain you would attend Duke if given the opportunity." When you say "YES!" (which you should do if that is the case), you will then be told you have 48 hours to send your deposit.
It is very obvious that Duke is managing their yield via waitlist. I know from a person there that their rankings have been hindered by lower yield compared to peer schools, which they are actively trying to increase. They get excellent marks on everything else, but yield is one place where they have lagged a bit, but are now starting to pick up.
If for any reason you gave the notion that Duke was not your top choice, like I somewhat did on my original application, you were most likely waitlisted. Example: my application was catered to finance, but I also applied to Booth (interviewed, but denied). Fuqua obviously inferred that Booth would most likely be my top choice (there were other factors I failed to mention, which is what I included in my updates) and therefore, waitlisted me. If you're serious about Fuqua, tell them straight up that you will go in the event you are admitted...yield is a huge thing in the rankings criteria, and they definitely take that into account.
Also, no WL movement happened on the 9th, it was on the 10th. I know this for a fact.