Apple Motion 3 - Manual (Page 550)

550 Chapter 6 Keyframes and Curves
Bezier handles appear and your mouse movement automatically controls one of the
handles.
Note: Command-clicking a Bezier point resets it to Linear interpolation.
To simultaneously modify the handles for more than one control point, Shift-click to
select multiple points, then adjust the tangents. If there are no tangents on the point,
drag it while holding down the Command key.
To break a handle:
m While holding down the Option key, drag the handle.
To "lock" the handle’s angle while modifying:
m While holding down the Option 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.