How does Authorize.Net Silent Post work?

Authorize.net offers a "Silent POST" feature for their Automated Recurring Billing. It's supposed to POST data to a url of your choosing, telling you whether they were able to charge the customer, how much, etc. The problem is, it isn't very well documented. - Is there any way to test a post to that URL? I've signed up for a developer account, but there's no way to specify that URL like you could in the actual system. Hence, there doesn't seem to be a way to test it out. - If not, is there a list of possible values it could return? It appears to send `x_first_name`, `x_amount` - I've seen code that uses those values - but since I can't actually get it to send a response, I'm not sure. - Is there documentation for this feature anywhere? Or even class that implements it fully?