So basically you can set it so that instead of downloading pieces randomly, it downloads them in order, then you can play the video file while it's downloading.

such sequential downloading is deemed to be quite harmful to the torrent network and protocol concept

wiki.vuze.com/w/Sequential_downloading_is_bad

Some argue that lots of people want it. While that may be true, BT protocol was not designed for streaming. Other protocols for downloading serve that purpose better.

That said, tixati does have some support for those need a sequential download now and then.