I didn't know until a few minutes ago, thanks to Nintendo's handy FAQ...
The Ambassador Certificate is a downloadable video available to Ambassadors.
By tapping the receive button on the Touch Screen when viewing this video, Ambassadors will receive updates regarding the Ambassador Program, such as when new titles are available to download. As soon as details for the 10 Game Boy Advance Virtual Console titles are available, we will send an update to those who have opted in using the Ambassador Certificate.
Ambassadors will be able to download 10 Game Boy Advance Virtual Console titles for free before the end of the year.
We'll have to keep an eye out for that! Thanks to Brandon for the heads up!