Apple Final Cut Pro 7 - Manual (Page 1441)

If neither an In nor an 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 playback in the QuickView tab.
Video is cached to RAM as it’s played. Once the portion of the sequence that you’ve
specified for QuickView playback has played all the way through, subsequent loops play
much faster because they’re being played back directly from RAM.
1441 Chapter 84 Using RT Extreme