The DVDs come from your queue. When Netflix receives DVDs from me, they ship the next two DVDs from my queue. If I have put a new release at the top of my queue, they ship it.
Sometimes there's a delay, but not usually for new releases. For example, I currently have "Patriots Day" as number 8 in my queue. It is marked "short delay." By the time I've received numbers one through seven, it will probably be available. You can rearrange, add and delete movies in your queue very easily. Added movies take the last spot in the queue. You can click and drag a movie to the top of your queue if you want to see it before the others.
Kudos to Netflix for having an affordable, easy to use and fast service.