[...]
FTP command issued as part of setting up a transfer with the connection direction being in the same direction as the command channel (i.e. the FTP server creates a temporary server socket and it is up to the client to connect to it.) Why do this? Generally--very generally--it's to get through a firewall. Why not just use HTTP? FTP is far faster than normally configured HTTP, and there are the usual legacy reasons, too.
As of February 2003, tcllib's ftp implements PASV, but ftpd does not.
http://compnetworking.about.com/library/glossary/bldef-pasv.htm
"The PASV ... verbs" [L1 ]
RFC 478 [L2 ]: "Server-Server Interaction"
RFC 1579 [L3 ]: "Firewall-friendly FTP"
RFC 959 [L4 ]: "File Transfer Protocol"
"The difference between PASV FTP and Normal FTP" [L5 ]