CrushFTP can be run as the reverse proxy server, or as a back end server with Apache in the front end (Reverse Proxy Apache).
To make CrushFTP be the reverse proxy, you need to have an entire sub-domain that you want proxied. For example, www.crushftp.com goes to one server, but www.crushsync.com could hit the same server, but be reverse proxied to another server behind CrushFTP.
These server preference settings are on the advanced tab of the individual port you are using for HTTP(s).
Using CrushFTP as a reverse proxy allows you to share a single front end server, while keeping back end applications servers secure. CrushFTP gives you some hammering protection where it can automatically ban abusive IPs. More functionality will be added to this feature as well in the future.
The reverse proxy also supports sub paths on a domain to be proxied, but if using this, any absolute links on the server running behind CrushFTP would need to be adjusted.