No extra support, just a very short stand so the center of gravity is lower (makes it less tippy). As far as creating something like this, it's just a bunch of flat rocks stacked very, very carefully.
The slate is arranged so the larger pieces are near the bottom, creating lots of ledges and hidey holes for the fish and shrimp. As you move up the tank the pieces get progressively smaller untill they are about an inch and half deep (front to back). With the slate "sloped" like this, the bulk of the weight of the slate keeps the larger pices at the bottom from tipping over, and the wall sort of "leans" on the back glass.
It's quite sturdy, and I even gave the tank a good shake and jumped around it to see if that would jar anything loose, so far so good. The only "cave ins" I had were during the initial stacking process, but once I had a stable foundation I had no more problems.
It just takes time, and a lot of experimenting to get the slate to stack nicely. Honestly I spent more time making it stable then I did trying to make it look good, but it worked out well in the end.