Apple Final Cut Pro X (10.0.9) - Manual (Page 80)

Chapter 6 Play back and skim media 80
4 In Final Cut Pro, choose Window > A/V Output.
The Viewer contents appear on the broadcast monitor. Viewer features (such as onscreen
controls and the title-safe and action-safe overlays) can be viewed only in Final Cut Pro.
Note: Video and audio are synced at the video frame (not audio sample) level.
To turn off A/V output, choose Window > A/V Output.
About background rendering
When you add effects, transitions, generators, titles, and other items that require rendering
before playback, the background render indicator (an orange bar) appears below the timecode
in the Timeline.
Background
render indicator
By default, background rendering begins 5 seconds after you stop working and moving
the pointer in Final Cut Pro. Background rendering continues while you work in a different
application. You can change this setting in the Final Cut Pro preferences.