Probably just before release. It strikes me as being similar to the contract terms when I sell an original work of fiction to a magazine, say -- part of what they pay for is the exclusive publication rights for a finite period of time, so I can't republish the story elsewhere until the exclusivity period (which can be as little as a month after publication but is typically more like 6 or 12 months) has expired. So I doubt it's even an "embargo," just a normal exclusivity window, albeit a long-ish one.
