How does this work with a typical podcast player? The media download is typically just an mp3. The podcast software maker would have to be in on the scam otherwise it's an mp3 that plays from beginning to end.
When your player requests the mp3 file in this example the server on the fly can generate an mp3 file that your player then downloads. This means that they can serve different ads in old podcast episodes or even remove them in old episodes.
I heard about this tech on a podcast actually.