Interesting - I took a look at how this works and why it needs a reference, and the answer is the usual one of the 'moov atom': a critical piece of metadata. Lo

Interesting - I took a look at how this works and why it needs a reference, and the answer is the usual one of the 'moov atom': a critical piece of metadata. Lots of programs output it at the end of the file, but that makes it vulnerable to truncation, and for streaming it's useful to move it to the beginning of the file. ffmpeg refers to this as "fast start".