Apple Final Cut Pro 5 - Manual (Page 1092)

Chapter 15 Previewing Effects and Comparing Frames 305
II
If an In point is set in the Timeline but no Out point is set:
Final Cut Pro caches video from the In point through the duration specified by the
Range slider for playback in the QuickView window.
If neither In nor Out point is set in the Timeline:
Final Cut Pro uses the position of the playhead, caching half of the duration specified
by the Range slider before the position of the playhead, and half after the playhead, for
display in the QuickView window.
Video is cached to RAM as it’s played. Once the duration of the sequence that you’ve
specified for QuickView playback has played all the way through, subsequent loops
play much faster since they’re being played back directly from RAM.