You can link to the relevant pages from Apache and Nginx:
* http://wiki.nginx.org/X-accel
* https://tn123.org/mod_xsendfile/

Again the previous explanation is a good bird's view about the feature. More details are available at the links above. The new text is confusing. It uses defined terms like `redirect` in the wrong context: (e.g. `allows a web application to redirect the request for a file to the web server`). And again, I don't think we need to link to that framework page. This is an advanced feature, if you want to use it, you should be able to get into the apache or nginx docs.