Live Playback is a part of the Playback settings.
The affect of changes in the Playback Panel
By default, changes in the Playback panel are like any other changes in the Shot Editor -- they do not take affect until you press the Go.
However, there are times when you may want to scrub and see the scrubbing in the Live broadcast.
Making your changes Live
By checking the "Live Playback" checkbox as shown above, the changes you make in the Playback panel become live.
This gives you fine tune control over the movie or audio in the Live broadcast.
NOTE: If you want to pause all movies in your broadcast, you can select "Pause All Movies" from the "Media" menu. This is quite useful if you only have one movie and you wish to quickly pause it.