On blend nodes, you have the option of inputting a source image from another node on the pipeline, effectively bypassing the pipes. For example, suppose you want to use the key-in clip matte in a colour correction in the Matte pipe of the CBlend node. Instead of copying all the nodes in the Front pipe of the MBlend node, you could just add a branch from the end of the MBlend Front pipe to the blue box on the CBlend node, as shown here.
When attaching an alternate input source to a Matte pipe, you should use a matte image. If you use a colour image, it will be converted to greyscale.
For instructions on drawing the branch, see Adding Nodes to the Pipeline.
