Apple Final Cut Pro 7 - Manual (Page 1095)

Adding additional keyframes increases the complexity of the effect, but the area in
between each pair of keyframes in your clip is still smoothly interpolated.
Determining the Number of Keyframes to Use
The complexity of the changes in your effects depends on the number of keyframes that
you add to a clip. You need at least two keyframes in a clip to make a dynamic change
from one value in an effects parameter to another. A more sophisticated change requires
three keyframes. To isolate a keyframe change to a certain section of the entire overlay
for an effect, you need at least four keyframes.
Creating Simple Effects with Two Keyframes
The simplest thing you can do to make a change is to add two keyframes. For example,
you can change the size, or scale, of a clip by adjusting its scale from 25 percent at the
first keyframe to 75 percent at the second keyframe.
Two keyframes let you
create a basic change.
1095 Chapter 67 Adjusting Parameters for Keyframed Effects