Keying - Nuke 101: Professional Compositing and Visual Effects - page 332

This now copies the alpha channel from the matte branch to the RGB channels
from the HueCorrect branch, which is exactly what you need.
The result of the ShuffleCopy node, though, is an unpremultiplied image. You
need this image to be premultiplied before you can composite it over the back-
ground.
9. Click ShuffleCopy1 and insert a Premult node after it.
Now you can use the output of Premult1 as the A input for Merge1.
10. Grab the top part of Merge1's A pipe and drag it to Premult1's output ( FIGURE
7.44 ) .
Figure 7.44. Moving an existing pipe from one place to another.