The Fast Start feature in QuickTime Pro allows you to begin playing a movie while it's still being downloaded. Instead of having to wait for the entire movie to download, QuickTime Player can start playing the already downloaded part, while the rest of the movie continues to download in the background.
For Fast Start to work, you need to select the Fast Start option in the Save dialog when you create the movie.
Please note that if you modify the movie after saving it with the Fast Start option, the Fast Start feature may not work correctly. For example, if you create a movie with Fast Start enabled and then add annotations and save the movie again, the Fast Start feature may not work.
To prevent this from occurring, be sure to save the movie again with the Fast Start checkbox selected before posting your movie for download.