From what I gather from the limited information about this tool, it intercepts the local PC's IP stack and masquerades the outgoing packets with a public IP address (presumably that of the WAN interface of the NAT router).
This is solving a problem that doesn't exist any more with modern routers.
The write up suggests that the client (or the NAT router) on the private LAN would be leaking it's private RFC1918 address to devices outside of its network.
Modern routers (in fact even NAT DSL routers I bought back in 2001, before that application
was written) present the router's public or WAN IP address(es) to remote services when outbound connections are made. If a FTP client behind a NAT router is using Active or Port mode, modern routers know that the inbound connection from the FTP server is for that particular client and will map the connection back to the client.
So unless you're using some ancient NAT router from the days of the ark, I wouldn't bother wasting any time with this.