How do I implement an expandable banner ad using ONLY Flash?

I've been tasked with developing an expandable banner ad for a website, but I've been informed that the alteration of HTML / Javascript on the websites part is not going to happen. The ad is supposed to expand over or push down the content beneath it.

From what I've found in my research, without leveraging Javascript, this is more or less impossible. The site claims they've implemented banner ads like this before without editing anything on their end. Is this even possible?

If not, what's the best way to implement these using Javascript. I've found countless different ways of doing it, and I'm wondering if one way is more widely accepted than another.

Thanks.