Actually, a "Path Forward Application" fetch the content of corresponding url in the target site together with the essential request parameters and return back the result to user.
It acts just like using CNAME in DNS server pointer to another address. However, the application can be expanded to handle url translation (i.e. change the url according to specific rules) or content filtering (i.e. remove or add specific contents according to some predefined rules).
To implement the test requests only some copy and paste work. The overall experiment can be done with a coupe of minutes. Here is the address: http://app-engine-guide.buss.hk/demo/path-forward-by-python-google-app-engine