Apple Final Cut Pro 5 - Manual (Page 1349)

562 Part III Output
Preserving Render Files
When you make changes to effects, composited media, and other edits that have
already been rendered, the render file for the affected item is no longer valid.
 If the project has been saved with the render file, the render file is deleted when
whichever of the following happens later:
 The project is saved again.
 The change falls off the “Undo queue” (the series of changes that can be reversed
with the Undo command in the Edit menu).
 If the project has not been saved since the render file was created, the render file is
deleted when the new change falls off the Undo queue.
Note: You can keep your important render files by nesting sequences (see the
next section).
Using Nested Sequences to Preserve Render Files
You can preserve render files for a sequence or for audio clips within a sequence by
nesting that sequence within another sequence. Nesting is particularly useful for
protecting the render files for effects-intensive clips you plan to trim frequently.
If a change is made to a parent sequence that’s been nested, it needs to be rerendered
before the nested sequence plays in real time. Modifications include any compositing,
speed changes, or changes in filters, transitions, or other effects.
For detailed information, see Volume II, Chapter 23, “Sequence to Sequence Editing.”
Disabling Tracks Affects Render Files
When you disable a video or audio track in the Timeline, any render files associated
with that track are lost. If this happens, you can restore that track’s render files by using
the Undo command.