Principles of a proxy FTP

I have recently started a programming project which is a proxy ftp (for now) using Python.

I have a small question kind of slowing me down though, I don't know what exactly happens when a client tries to connect to an ftp server through a proxy. The RFC is great help to understand how FTP works normally but doesn't give any hints about the process if a proxy is used.

So what is happening? I would assume that the client tells in some way to the proxy what ftp server its trying to reach...but how? I've checked quite a few forums and haven't found any information about that process. I think I will try sniffing the traffic while using some proxy, might help a bit.

Anyway if anyone has experience in that field that they don't mind sharing, i would be glad to know!