The way I read it, it's a PyPy-the-VM feature, used in RPython, but not a PyPy-the-interpreter feature callable in the Python code interpreted, hence it's not a

The way I read it, it's a PyPy-the-VM feature, used in RPython, but not a PyPy-the-interpreter feature callable in the Python code interpreted, hence it's not available to users (i.e Python devs)