Some necessary changes had to be made from how the portals usually operate. For example, there's a timer for each portal, instituted in order to keep players from being stuck in perpetual loops. Also, by reloading, you can manually kill any active portals in the game world.
Peer past the break to check out a bunch of videos showcasing the mod, courtesy of 3 Point Games and Pixelated Geek.
[Via Pixelated Geek; thanks, Jessica]