This option is very helpful when your drive surface is located between two surfaces. Especially the impeller machining with its twisted turbine blades becomes very comfortable.
The main advantage is the possibility to compensate the tool to the drive surface and check surface in the left and right corner of the work piece. All you need is the tool radius which is the distance (margin) between the tool center and the surfaces. It should be the radius of your tool.
Important in this function is:
that the tool has to be a sphere mill and
the drive surface edge has to be the check surface edge (so the check surfaces encloses the drive surface).
If "Morph between two surfaces" is selected you have to pick the first and the second check surface by pressing the buttons. Pick these two surfaces embedding the drive surface.
The area type has to be set to Full, start and end at exact surface edge because the distance between margin and first cut depends on the exact position of the surface edge.
Select the "Advanced…" button and the margins window pops up. Here you can enter the distance values. The start margin belongs to the first surface and the end margin to the second surface. The value should be the radius of your tool.
If you use ball cutters and would like to have double tangency (pencil tracing), you have to switch on Calc. based on tool center in the Utility tab. If the calculation is not based on the tool center, an incorrect toolpath will be generated.
This example shows an impeller section. The tool diameter is 6 mm. So the start margin is 3 mm. As you can see the distances between the drive surface and the check surface to the sphere center of the tool is 3 mm.
This is also essential for the end surface.
See Advanced Options.