To be more accurate, the document will not start, including showing the first scene. This is why Preload is an option in the Resource Library.
I assume you are referring to the attribute for the HTML5 <video> tag? Autobuffer is a deprecated form of preload. I don't think you need it unless you care about really old versions of browsers, but you may want to research it more. Hype doesn't use it (and I never have).