Apple Shake 4 - Manual (Page 419)

Chapter 15 Image Processing Basics 419
Example 3; Assigning an Alpha Channel With the SwitchMatte
Node
In the following example, the mask is drawn using the QuickShape node and copied in
as the alpha channel for the bus via the SwitchMatte node. Because no color corrections
have been made, the MatteMult toggle is used in SwitchMatte to premultiply the
foreground.
The bus is color corrected in the next example, so preMultiply is disabled in the
SwitchMatte node, and enabled in the Over node. Or, you can also insert a Color–MMult
node between SwitchMatte1 and Over2.
Tree with SwitchMatte node Matte bus (no alpha) QuickPaint
Background Over result Premultiplied result
of SwitchMatte node