Apple Motion 4 - Manual (Page 367)

Insert
When you choose Insert from the drop menu, Motion leaves the existing object in the
track, but pushes it forward in time to make room for the new item. For example, if you
insert a five-frame movie into a group containing an existing object, the new movie is
added to the Timeline at whichever frame you drop it, pushing the remaining frames of
the original movie out five frames.
Before After
If you insert an object midway through an existing object, the existing object is split into
two objects, each on its own track.
To insert an object
1 Drag an object from the Library or Media tab into the Timeline track area.
As you drag, a tooltip appears at the pointer to tell you the frame number at which you
are located.
2 When you reach the frame where you want the new object to start, keep the mouse
button pressed until the drop menu appears.
3 Choose Insert from the drop menu.
The new object is inserted into the track, breaking the original bar into two, and pushing
the frames after the insertion further out in time.
Overwrite
The Overwrite drop menu option deletes the existing object, overwriting it with your
new object.
Before After
367 Chapter 8 Using the Timeline