...yeah, it does:

This supplies code that can manipulate the returned value from the action for a specific rendering type (eg: JSON, YAML, or console).  This allows you to work around limitations of the underlying object that the action returns, or perhaps to produce a more human-friendly output.

You should not be using this.  Any action that requires a `when_rendering` hook is returning the wrong object to the world; the returned object itself should be able to correctly render to whatever format the user requests.