Apple Shake 4 - Manual (Page 566)

566 Chapter 20 Rotoscoping
Important: Be careful with the soft edges—if you create a shape with overlapping
lines, rendering artifacts may appear. To clean up minor artifacts, apply a slight blur
using the Blur node.
Linking Shapes Together
When you right-click the transform control, you can set up a skeleton relationship
between your shapes. Right-click and choose Add Child from the shortcut menu, then
click the transform control of the shape you want as a child of the current shape. To
remove the link, right-click, then choose Remove Parent from the shortcut menu.
Once a link is established, modifying a shape affects its children.