Apple Motion 4 - Manual (Page 385)

You cannot trim the individual object tracks within the body of the group track.
Before After
Disconnecting the Group Track from Its Contents
You can modify the group bar to be longer or shorter than the contents of the group.
For example, you might shorten a group bar to hide a section of the objects within it.
Objects that extend beyond the boundaries of the group bar (the thin blue bar at the
top of the group track) are not displayed in the Canvas.
Before After
To change the length of the group independently from the objects within it
µ
Press Command, then drag the edge of the group track.
Only the thin blue layer bar is trimmed.
Note: Once you manually change the length of the group bar, it is no longer automatically
updated as you add or modify the objects within it. To restore automatic updating, realign
the group bar with the right edge of the last clip in the group.
You can use this same technique to trim an object without automatically changing the
duration of masks, filters, or other applied effects.
385 Chapter 8 Using the Timeline