how to use and what is the use of tcpkill command

tcpkill command:
Use tcpkill command to kill specified in-progress TCP connections. It is useful for libnids-based applications which require a full TCP 3-whs for TCB creation.
Syntax:
tcpkill -i eth0 { expression }
Examples:
(a) Kill all outgoing ftp (port 21) connection:
tcpkill -i eth0 port 21
(b) Kill all all packets arriving at or departing from host 192.168.1.2 (host12.nixcraft.com)
tcpkill host 192.168.1.2
OR
tcpkill host host12.nixcraft.com
(c) To kill all IP packets between 192.168.1.2 and any host except 192.168.1.111, type the following:
tcpkill ip host 192.168.1.2 and not 192.168.1.111