Sometimes called FREB, the request-based tracing provides a good way to figure out what exactly is happening with your requests and why, provided you can reproduce the problem you are experiencing. Problems like poor performance on some requests, or authentication related failures on other requests, or even the server 500 error from ASP or ASP.net can often be very difficult to troubleshoot--unless you have captured the trace of the problem when it occurs.
The following video shows how to configure trace on any IIS7 site once the feature has been enabled.
http://channel9.msdn.com/posts/dunnry/Failed-Request-Tracing-on-IIS7/
More details on Tracing may be found here.
http://www.iis.net/ConfigReference/system.webServer/tracing
Please note that remote administration must be enabled in order to perform a trace on your site via the IIS Management Console.