Adobe After Effects CS4 - Manual (Page 266)

260 USING AFTER EFFECTS CS4
Animation and keyframes
Last updated 12/21/2009
Overlap pin with negative In Front value (top), and Overlap pin with positive In Front value (bottom)
Each Overlap pin has the following properties:
In Front The apparent proximity to the viewer. The influence of Overlap pins is cumulative, meaning that the In Front
values are added together for places on the mesh where extents overlap. You can use negative In Front values to cancel
out the influence of another Overlap pin at a specific location.
An area of the mesh that is not influenced by Overlap pins has an implicit In Front value of 0. The default value for a
new Overlap pin is 50.
When animating the In Front value, you should usually use Hold keyframes. You do not usually want to interpolate
gradually from an element being in front to an element being in back.
Extent How far from the Overlap pin its influence extends. The influence ends abruptly; it does not decrease gradually
with distance from the pin. Extent is indicated visually by a fill in the affected parts of the mesh. The fill is dark if In
Front is negative; the fill is light if In Front is positive.
Puppet Starch controls
When you are distorting one part of an image, you may want to prevent other parts from being distorted. For example,
you may want to preserve the rigidity of an arm as you move a hand to make it wave. Use the Puppet Starch tool to
apply Starch pins to the part of an object that you want to keep rigid.
You apply Puppet Starch pins to the original outline, not to the deformed image.