How Does A Preloader Affect Movie Download?

Here's something that might be an elementary question for you all. How does a preloader affect the movie download? I thought that one of the advantages of flash was that it started playing while it downloads. Does a preloader stop that from happening? Any brief but complete explaination would be appreciated.

Thanks in advance!

Downloading is not affected by a preloader. All the preloader does is allow you to display something to the user until all or some of the movie has downloaded.