Not exactly sure, but playing with setting up your own VPN will give you an idea of it.

Essentially, the VPN is run on a remote server. When you connect to the VPN, your traffic gets masqueraded out through the remote server, and replies get natted back to you. If you tried setting up a webserver on your computer and then accessing the webserver on the VPN server IP, it wouldn’t work, because the request coming in to the VPN server port would by default just reach the VPN server at that port.

This is where port forwarding comes in – if the VPN server allows you to port forward, you can set port X on the VPN server to go to port Y on your router (which would likely also have to port forward on your router to get to your computer).

If you were intending to seed, yes.

It’s certainly possible to download content after you’ve joined. Some might call that piracy, but I would call it fair use.

Port forwarding allows you to seed without revealing your IP address. If you seed on a VPN and you don’t have port forwarding, your IP is revealed to those you seed to.

Edit – I was wrong. You can technically seed securely behind a VPN connection, but since you can’t be connected to directly behind a vpn without port forwarding, you would only be able to seed to ppl who port forward, or who can be connected to directly. You would only be able to leech from everyone else.

If everyone torrented like this (behind a VPN without port forwarding), all torrents would grind to a halt; nobody would be able to seed.