Port Magic runs in your Windows task bar and listens for applications like instant messaging and multi-player games that try to open ports to the Internet. When a program tries to communicate with the Internet, Port Magic will ask if you want to allow the program to open a port on your router. If you say yes, Port Magic will automatically configure your router to allow the program's communications to pass through to your computer. If you say no, a port will not be opened.
You can see all of the programs that have asked to open ports in Port Magic's Applications window. From this window, you can change any of the settings for each program that is listed.