Apple Final Cut Pro 5 - Manual (Page 569)

234 Part II Rough Editing
2 Choose Modify > Mark in Sync.
The items are now marked as in sync, although their positions in the Timeline
haven’t changed.
If you select just the audio and move it out of sync, out-of-sync indicators appear.
The Mark in Sync command permanently affects the sync relationship of the selected clip
items in your sequence. Once you’ve modified the sync relationship of clip items, the only
way to restore the original sync relationship is to manually move the clip items into the
old sync relationship and use the Mark in Sync command again, or to delete the clip
items and reedit a new instance of that clip into your sequence from the Browser.
The out-of-sync
indicators disappear.
The out-of-sync indicators
show the new offset, not
the original offset.