Setting Alpha Channel Options
Adding a motion setting to a clip frequently means that you’re also superimposing it over other clips in the program. Conveniently, the Motion Settings feature supplies an alpha channel that you can use to key out the parts of the frame left empty by the moving clip.
However, if the clip already contains an alpha channel, you must specify whether to use that channel or the alpha channel generated by the motion setting. Do this by selecting an alpha channel option in the Motion Settings dialog box.
To select an alpha channel option:
In the Motion Settings dialog box, select the appropriate Alpha radio button:
Create New— to create an alpha channel that matches the frame of the clip, ignoring any existing alpha channel ...