As I understand the process, once you set up the Slingbox connected to your friend's router on their local area network, and setup your account on the Slingbox website, then the Slingbox will communicate periodically with the Slingbox directory to keep the IP addresses (external for their modem and internal for your Slingbox within their router) up to date. Then, when you visit the website http://watch.slingbox.com from a computer anywhere in Internet land and login in with your ID and password, the directory of Slingboxes in the directory at the Sling Media directory will connect with that address combination and the Slingbox will send out the video and audio to the IP addresses (external for your modem and internal for the local area network that your computer is connected to at the moment). A web browser add-on to display the audio and video in your web browser will be automatically installed the first time you make the connection.
And since routers sometimes have a tendancy to assign new IP addresses dynamically to the devices attached to them when they reboot (power outages and the like), the Slingmedia staff recommend that you program the router where your SB is physically located to assign a static IP address to the SB, and then program the router to forward port 5001 to that static IP address, so as to make this "address handshake" between the SB and the Sling Directory more stable (and, hopefully, never changing -- unless, of course, your friends change their Internet Service Provider). If the IP address of the SB gets changed from the one in the Slingmedia directory, your friends may have to physically push the reset button on the back of the SB to get the new local IP address communicated to the Slingmedia directory.
So long as the internet service where your Slingbox is physically located has adequate UPLOAD speed, and the internet service where you are viewing the video on your computer has adequate DOWNLOAD speed, then you should be able to view the video smoothly. The specifications pages on the Sling Media website provides the recommended minimum internet speeds for smooth viewing.
What I understand is that you would like to know about the Slingbox Internet Viewing.
In this case, you can check the following link about it and how it works.