Apple Final Cut Pro 7 - Manual (Page 1440)

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.
1440 Chapter 84 Using RT Extreme