Just Cause 2 is a study in vehicular insanity--few games let you leap from a plane in mid-flight, pull out your grappling hook, and zip into the cockpit of a different plane. That grappling hook, in fact, leads to some of the most insane, nearly game-breaking moments you can have with Just Cause. You see, the grappling hook can attach to anything, letting you zip toward an object or attach two things together. This leads to pure chaos in single-player.
Now imagine what grappling hooks can do in the hands of dozens of other players. People hijack speedboats, attach cargo to planes as they take off, and chain their friends to moving vehicles--that's just part of what this multiplayer mod allows. Others get into vehicle races, but these quickly devolve into clusterfraks of people latching onto the jeep in front of them, while racers in jets above just crash into each other. It's basically Michael Bay's wet dream.