Port forwarding is a feature to establish a route from within one network to another network. Imagine Mullvad is acting as your router. You used to be able to go into Mullvad’s website and get a forwarded port. This means you could set your torrent client’s port to match what Mullvad gave you, making your torrent client directly reachable to anyone on the internet.

Without the port forwarding feature, you can only connect to other “reachable” torrent clients that are directly exposed to the internet.

If you have never set up this feature from Mullvad’s site before, you will not experience a change in anything. But the benefit with a forwarded port is that you can communicate to more peers in a swarm, because you can connect to both reachable and non-reachable clients. Technically, it’s the non-reachable ones who can reach you.

For well-seeded and alive torrents this doesn’t matter to much. For creating new torrents and sharing them, or connecting to nearly dead torrents, this sucks.

Also, port forwarding isn’t just about uploading. It’s just about establishing connections.