frevvo can POST form or workflow data to your web server for processing. There are two options available Form Action Post and Doc Action Post. The difference between the two is that the Form Action POST displays the response from the web server to the user whereas the Doc Action POST does not.
HTTP Wait-Notify is a step, configured with a post url, that can be added to a workflow. The workflow data is posted to this url when the task is executed. A call back url is included in the post. The workflow and task are suspended until the receiver posts back.
Processing form data POSTed to your web server is described in the Submissions Stored Outside of frevvo.