This is one of the most fascinating things about PyPy, and one least understood (most likely because the primary output of the PyPy interpreter generator happens to be a Python interpreter written in a static language much like Python called RPython).

I've been toying with the idea of building a little language, just for fun. The mess of building an interpreter has always kept me from it, but as I've spent some time looking at PyPy, I think I might just have a go.