Vcarve does not show tool undercut. It will only show the slot in the simulation, but it works. The gadget does draw a sample on the 2D View screen. Test a cut on a scrap board until you're comfortable. I set my XY Datum position to center, so I know exactly where the cut starts. I just set the bit XY position to where I want the big part of the hole to start, and Z to the surface of the board. I have two saved files I generally use; left to right and bottom to top. That pretty much covers all my needs. Clamp it well. Lots of side force when the bit cuts.