FXP works by simulating pasv upload/download on one site and "active" download/upload on the other.
in order for FXP to work sites must allow for PORT command to have ip other than the one used during login and must accept incoming data connections from ips other the one used during login.
it is possible for a site to allow only port w/ other ip OR incoming connecitons from other ip, however with most ftpd if FXP is disable, both of these "features" are disabled.
i don't want to sound like i'm brushing you off, but consider reading some ftp RFC's. there are not extremely complex and will let you understand what exactly is going on during ftp/fxp session.
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