I don't know the details, but a bit torrent client can request arbitrary blocks from its peers. If this wasn't possible, doing things like resuming a partial do

I don't know the details, but a bit torrent client can request arbitrary blocks from its peers. If this wasn't possible, doing things like resuming a partial download, or only downloading a single file from a .torrent that contains many files would be impossible. I guess this app just requests blocks starting from the beginning of the movie, and once it has enough buffered, it starts playing.