Apple Motion 3 - Manual (Page 546)

546 Chapter 6 Keyframes and Curves
When you set the interpolation for a curve, you select the keyframe you want to
modify. The method you choose determines the distribution of values through, into, or
out of the selected keyframe.
To set an interpolation method for a keyframe:
1 Select the keyframe you want to affect.
The keyframe turns white.
2 Control-click the keyframe to display the shortcut menu.
3 Choose an interpolation method from the Interpolation submenu.
To set an interpolation method on a curve segment:
m Control-click the segment, then choose an interpolation method from the Interpolation
submenu.
You can set different interpolation methods for different segments of the same curve.