I just downloaded a linux image using bittorrent. Although the port seems to be closed I noticed that my client was also uploading. As far as I know UPNP and NAT-PMP are disabled in my current network. Does my client (Transmission) utilize other techniques like TCP-hole-punching? How is uploading with closed ports possible?
Your computer is connecting to other computers through the tracker. Because your computer is initiating those connections, they go through even though the port is closed. You would get more connections to upload if the ports were not blocked.
Most people would say you should open the ports so you can help others download what you did.