Managing Cycles Materials - Blender 2.6 Cycles: Materials and Textures - page 61

19. Go out of edit mode and select the outer Mix Shader ; press Alt + D
(this shortcut removes a node from a network leaving the connection un-
touched) to disconnect it. Delete it.
How it works...
I think you get the picture: basically any input or output socket of the nodes wrapped
in a group can be piped to the outside of the node group itself to be tweaked; for
example, we could also have made an input socket for the Diffuse roughness, or
better, an input for the Fac of the inner Mix Shader , to change the reflectivity of the
shader.