Waldo, the advantage of the Gemini plugin is that you can work on a symmetrical object without worrying about what will weld and what won't - you can even adjust the tolerance to whatever you choose. This hour glass makes use of that feature to create the waist between the two halves. Obviously a model like this could be made pretty quickly without Gemini but making a model of (say) a human head would be much more time consuming. The trial will only work in the Z Negative direction so leave the 'Keep' setting at that. When you have a shape you're happy with go to the Edit menu in the Assembly Room, choose 'Gemini' and then you can join your two halves to form a new Vertex model. You can then smooth this if you want. It's worth persisting!