Apple Final Cut Pro 6 - Manual (Page 547)

214 Part II Rough Editing
When video and audio clip items are linked in the Timeline:
 The names of the linked clip items are underlined to indicate that they’re linked.
 As long as linked selection is on in the Timeline (the Linked Selection button in the
upper-right corner is green), clicking one clip item selects it and all the items
linked to it.
When Linked Clips Are Moved out of Sync
When you move clip items in the Timeline, Final Cut Pro checks to see if the
relationship between linked items is still correct. If the relationship does not match,
Final Cut Pro displays out-of-sync indicators in the Timeline. Out-of-sync indicators show
the offset between the linked clip items.
The underlined clip name
indicates the link between
audio and video items.
Click the Linked
Selection button to
turn linked selection
on and off.
Out-of-sync indicator