Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 77

Now that you copied the red channel into the alpha channel, Merge2 works and
the doll's feet now appear to be behind the wooden box ( FIGURE 2.49 ) .
Figure 2.49. That's more like it. The doll's feet are now behind the front of
the toolbox.
Viewing a composite without rendering
Let's look at your composite so far. To do that, you need to render your tree as a flipbook
into Framecycler.