The idea is that you start with a basic rezzer block, and click it to begin. It will rez a white block for you, which you can move off wherever you want it. Another white block will appear over the rezzer -- click it, and then click any face of the other cube, and it will jump straight to it. When you use a cube, another appears for you. You can use it to build up shapes by joining cubes together in various arrangements. I was never much of an artist, but I reckon I can do immersive cubism with this! :-)

There's lots of potential variations on the idea. I would probably choose to make it work by 'growing' your cubic construction from the root prim -- you click a side, and it rezzes the next cube in the appropriate location. That's less flexible though, since it's more difficult to move existing cubes then.