Port knocking is an authentication technique used to open closed ports which are behind a firewall. It is mainly used to prevent hackers from doing a port scan and is one of the security measures a network administrator takes to secure the network.
Torrent download is based on the Peer-to-Peer network where all downloadable files are stored in different nodes rather on a dedicated host. To initiate a torrent download, you will need a torrent download client. Some of the popular torrent clients include qBittorrent, Deluge etc.
Peer-to-Peer (P2P) file sharing is a type of file sharing system which enables sharing content through a distributed architecture of computers. Unlike other architectures, it does not need a central server and instead enables users to communicate with each other directly.