Casper described the process in a comment on MOCpages:
The technique is that you start out with styrofoam and dig a hole with a very hot thingy. Then you place a Minifig in the midle and place the whole thing in castig and make a molding form(?). Then you burn this in a very hot oven as to make everything plastic and organic to burn away. Then you pour bronze into the mold and wupti – this is what you get.
Contrary to some of the horrified responses, I think this is a beautiful piece of LEGO-inspired art.