Apple Motion 4 - Manual (Page 368)

If the new object is shorter than the one currently in the group, the Overwrite option
splits the duration of the existing object and deletes only the frames where the new
object appears.
Before After
To overwrite an object
1 Drag an object from the Library or Media tab into the Timeline track area.
As you drag, a tooltip appears 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, hold down the mouse
button until the drop menu appears.
3 Choose Overwrite from the drop menu.
The frames of the new object replace the frames of the original object. If the original
object contained more frames than the new one, the old object is split into two objects
and the additional frames remain.
Exchange
The Exchange drop menu option is a variant of the Overwrite option, but instead of
dropping the entire duration of the new object into the project, the duration of the
existing object is used.
For example, if you drag a 30-second clip over a 5-second clip, choosing Exchange swaps
the existing 5 seconds with the first 5 seconds of the longer clip.
Before After
Similarly, if you try to exchange a 10-second clip with one that lasts only 5 seconds, the
first 5 seconds are replaced, and the remaining 5 seconds would remain. The Exchange
option transfers any filters, behaviors, and keyframes from the original object onto the
new object.
368 Chapter 8 Using the Timeline