Apple Motion 4 - Manual (Page 586)

To “lock” the handle’s angle while modifying
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While holding down the Shift key, drag the handle.
Extrapolation
In addition to setting interpolation for the areas between keyframes, you can define how
Motion generates the values before the first keyframe and after the last one (extrapolation).
When you set a such a method for a parameter, new keyframes are added beyond your
original keyframes. This can be helpful when trying to extend the duration of an effect
such as a moving background.
To apply the Before First Keyframe setting for a parameter
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In the Keyframe Editor, open the Animation menu for the parameter you want to change,
then choose an item from the Before First Keyframe submenu.
586 Chapter 10 Keyframes and Curves