Ah, I've been doing stuff like that about few months ago. Looks like the guy has no idea about Shift+H combination, poor soul. The whole thing about it is precise positioning and a bit of maths (loled when he used the calculator to count angles). Arcs are a separate style of terraining, giving lots of freedom in city areas, but also a lot of laggs if overused. Arcs are multy-use props, you can make buildings out of them, you can make bridges out of them, so made robots or giant stone rings (refering to "Legendary Terrains")