That’s a built-in feature of FTP that doesn’t require server support.

Edit: Source: https://en.wikipedia.org/wiki/File_eXchange_Protocol#Technic...

1. You connect to servers A and B.

2. Tell B to receive a PASV transfer. It replies with the IP address and port it's receiving on.

3. Tell A to send to that address and port.

This is documented in RFC 959, starting with

  "In another situation a user might wish to transfer files between two hosts, neither of which is a local host."