Most consumer VPNs use a NAT firewall to protect their users against unwanted and potentially malicious incoming traffic. This is beneficial most of the time, however, when it comes to torrenting peer-to-peer, it can cause issues. This is because a NAT firewall blocks all traffic; including incoming torrent requests
Most of the time, it will not matter if one user is behind a NAT firewall because modern BitTorrent clients are designed to initiate the connection between clients directly. This usually serves to bypass the NAT. However, if both users are behind a VPN’s NAT Firewall, it can become impossible for the torrent client to establish a connection.
This becomes problematic when many users are all behind a VPN, which is becoming more common nowadays. In order to seed file pieces successfully all the time, it is better for your VPN to allow incoming traffic to reach your Transmission client. To permit this, it is usually necessary to set up port forwarding. This is a special VPN feature that allows users to assign a port for routing traffic through the VPN’s NAT Firewall and to their BitTorrent client.
To make port forwarding easier to set up, Transmission supports NAT Port Mapping Protocol (NAT-PMP) and Universal Plug and Play (UPnP) port mapping. This allows you to automatically set up port forwarding, which makes things easier than on alternative BitTorrent clients that require you to manually specify an open port.
Whether you think you will need a VPN with port forwarding is largely your choice. Many internet users torrent without issues using a regular VPN connection with no port forwarding, and approximately 90% of torrents should finish downloading even without the use of port forwarding.