Apple Final Cut Express 4 - Manual (Page 323)

Chapter 23 Setting Edit Points for Clips and Sequences 323
VI
When You Set One Sequence In or Out Point
If you set only one In or Out point, that point determines where the clip being edited
into your sequence will start or end:
 If you set a sequence In point, the In point of the source clip is placed at the sequence
In point, and the clip extends from the In point to the right for the duration of the
source clip.
In point
The new clip begins at
the In point that you set.