I’ve been working on some involved features lately, so the updates are little delayed. One of such features is “Mirror”. It is only partially implemented as yet.
A bulk of models are symmetric. So every 3D modelling tool has to make that task super easy. That’s achieved in 3DTin by this feature “Mirror”. Here is an example.
I haven’t yet worked out how the UI will work in order to use this feature, so even if it’s deployed in semi-working order you can’t directly use it.
[If you still want to, here is how to do it. Add a cube. Select it. Press “Ctrl+k”. This causes the cube to be bisected at the center and an invisible mirror gets added in that plane. Switch to EDIT mode (press TAB) and try changing the geometry. You should see it reflected on other side.]