Customizing Nuke with Gizmos - Nuke 101: Professional Compositing and Visual Effects - page 519

From the Pick Knobs to Add panel, you can expose any Knob from the tree held
in the Group. You just need to find it. You're looking for a node called Merge2
and a property called Operation.
8. Navigate and open submenus until you find the Operation property, as shown in
FIGURE 12.36 . Then select it and click OK.
Figure 12.36. Finding the right property requires a little digging.