Landing today in Pitivi Git is proxy editing support. Proxy editing is implemented with the help of GStreamer's GstTranscoder for transcoding the media content for proxy purposes. Pitivi proxy editing is described in detail via their requirements page and via this feature enhancement request.
Proxy editing basically comes down to interacting with lower-quality video clips to allow for a more responsive user experience while when it comes to rendering the final cut that it automatically switches over to using the original, full-quality media content. This is particularly useful if on a slower system where editing HD video footage in real-time would be painful where as proxy editing would allow you to interact easier with a lower quality version while the final result would still be the original quality.
This change also makes Pitivi use an intermediary format while doing the editing, which the developers desired to help work towards Pitivi 1.0. The current list of work for targeting Pitivi 0.6 can be found here.