I have found that my connection decreased suddenly when downloading torrents. I diagnosed it with bit torrent and the answer is that i should do port forwarding on my modem/router! Is it risky?
Port-forwarding means that when a connection is attempted on a specific port on your router, that connection will be forwarded to you (a machine behind the NAT) on that specific port (or, depending on the setup, it can be a different port, but let's not get into that).
Is it risky? Well, it can be risky. It all depends on the application listening on that port. In this case, the application is your BitTorrent client. There are two factors here:
Your trust in the BitTorrent client: You trust that the BitTorrent isn't actually opening a backdoor to your machine.
How up to date you're keeping your BitTorrent client: Outdated clients can have certain upatched vulnerabilities with publicly available exploits.
If you have those two points in check and you think you'll benefit from opening port (I don't think your speed will increase), then go ahead and forward that port.
Port Forwarding is just a technique used to enable "proper" network connectivity for some applications that are running on NATed devices (almost all home computers).
Port forwarding in itself a risk factor - the risks occur with regards to how the destination computer handles the incoming connections. e.g. If you forward port 22 (SSH), make sure you have a strong password, keep the SSH server well patched, and review any failed login attempts.
For some BitTorrent clients, port forwarding increases performance. Since most BitTorrent clients use random high ports that aren't used for other services, I'd say that there is little to no risk to enable port forwarding of whatever 5-digit port number that the client specifies.
In my opinion, the risks involved in executing files acquired via BitTorrent far outweigh the risk from forwarding a high port used by the BitTorrent client.
You should (at the very least) virus scan any such files before opening them.