Apple Final Cut Pro 5 - Manual (Page 882)

Chapter 6 Using the Audio Mixer 95
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8 When you’ve finished making adjustments, stop playback.
If keyframe recording stops before the end of a clip, the audio or pan level of the
remaining duration of that clip depends on whether or not there were already
keyframes in the clip.
 If there were no keyframes in the clip prior to recording keyframes: The audio level or
pan overlay remains at the level of the last recorded keyframe for the remaining
duration of that clip.
 If there were keyframes in the clip prior to recording keyframes: The audio level or pan
values are interpolated from the level of the last recorded keyframe to the level of
the next keyframe.