Apple Motion 3 - Manual (Page 332)

332 Chapter 4 Using the Timeline
Note: If you release the mouse button before the drop menu appears, the default
choice of Composite is applied. This means that the new layer appears on top of other
objects in the Canvas.
Composite
When you choose Composite, the new object is added to a new track within the active
group and all layers remain onscreen simultaneously.
Insert
When you choose Insert, 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.
If you insert an object midway through an existing object, the existing object is split
into two objects, each on its own track.
Before After
Before After