Apple Final Cut Pro 6 - Manual (Page 1427)

642 Part IV Real Time and Rendering
Playback in the QuickView Tab
QuickView playback relies, in part, on the amount of memory allocated to Final Cut Pro.
Based on this allocation, Final Cut Pro caches to RAM as much of the sequence
between the In and Out points set in the Timeline as possible for accelerated playback.
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 tab.