Question for the developpers: How does utorrent manages port binding?

[chillhood]

I have a router and I put rules to open a port for utorrent to use when running. My router redirects its port 40460 to my computer's port 40460. I noticed that when I have utorrent running, if I go on the port checker test page https://www.utorrent.com/testport3.php?port=40460 I'll see that my port is open but as soon as I close utorrent if I go back on the port checker page it'll say that my port isn't open. I guess it's normal behaviour so I was wondering how does utorrent manages to open the port on my router dynamically ? It intrigues me because I also have the filezilla server port redirected in my router nat rules. but when I check that port it's always closed whether the filezilla server is running or not. if you know something about it feel free to answer me even if you are not a utorrent developer ;-)

another thing I noticed: if I choose to not limit my upload speed it considerably slows my internet connection down. page loading and stuff. it's really drastic and flagrant. you can see how the 2 curves (up(red)/down(yellow) bandwith in task manager) cross each other as soon as I do that...why does that happen ?

[Switeck]

If you don't limit uTorrent's upload speed, it tries to upload at unlimited speed...that causes lag.