[Yoda]

Only 'non-dual' horizontal and vertical when the clubhead COVERS the plane line during the entire stroke...correct?

That is correct, Matt.

The Clubhead Covers -- not Traces -- the selected Delivery Line with both Horizontal (Only) Hinge Action and Vertical (Only) Hinge Action. Thus, the Clubhead remains in a Vertical Plane -- not an Inclined Plane -- as the Clubface Closes Only (Horizontal Hinge Motion) or Lays Back Only (Vertical Hinge Motion).

Notice that I have used Motion as a differentiating term here. The Hinge Action as executed by the Flat Left Wrist (remaining Vertical to either the Horizontal or Vertical Plane) produces a corresponding Hinge Motion of the Clubface.

"Precision is recognizing and reconciling minute differentiations."

-- Homer Kelley (2-0)