canceling/redirecting realm in pre-proxy ?

There seems to be some comments about being able to "cancel" a proxy in the pre-proxy section.. # When the server decides to proxy a request to a home server, # the proxied request is first passed through the pre-proxy # stage. This stage can re-write the request, or decide to # cancel the proxy. What I really want to do is test some variables (unlang) and based on the outcome, I want to actually handle the request locally rather than proxy. Maybe this is obvious, but I am not seeing it ? Thanks, Robert