Apple Motion 4 - Manual (Page 582)

You can set different interpolation methods for different segments of the same curve.
When different interpolation methods are applied to the different segments of an
animation curve, the methods that are used in the curve appear with a dash next to their
name in the Interpolation submenu (in the Keyframe Editor parameter list). In the following
image, all interpolation methods are applied to the selected curve.
The different interpolation methods are described in the table below.
Description Example Interpolation method
Holds the keyframe at its current value when
applied to a segment or keyframe and then
changes suddenly to the new value when the next
keyframe occurs. Constant
When applied to a keyframe, creates a uniform
distribution of values through the keyframe from
its two adjacent keyframes. When applied to a
segment, creates uniform distribution of values
between the two points. Linear
582 Chapter 10 Keyframes and Curves