Trivia: The scene in which Julia brings Ethan back to life is copied almost identically from the scene in "Lost" (Director JJ Abrams' hit TV show) where Charlie is brought back to life. Incidentally, you can hear the "Lost" music played throughout the show, most noticeable when Davian is about to pick up the gun to kill Julia.
Trivia: One of the reasons why they chose the HK G36 as the rifle Ethan uses in the bridge scene is because it made the scene with Ethan throwing the rifle over the hole blown in the bridge possible in the real world. The G36 is maybe the only rifle in the world that is purposely designed to endure a 2m fall without misfiring. Any other rifle would have ejected its magazine if dropped.
Trivia: The hands of the coroner removing the explosive charge from Lindsey Farris' dead head are that of the film's director, J.J. Abrams.
Trivia: When Ethan answers the phone at the party and receives the call about a free trip to Mexico, the voice on the other line is that of the film's director, J.J. Abrams.
Trivia: After the release of "Shaun of the Dead," Simon Pegg was asked in an interview if he would continue making movies in Britain, or if he would try to build a career in Hollywood. Pegg jokingly blurted out "Well, I'm not about to go and star in Mission: Impossible III." Shortly after the interview was printed, Pegg was indeed offered a role in "Mission: Impossible III," and he has since appeared in every subsequent film in the series.
Trivia: This was the first movie J.J. Abrams ever directed. He had previously directed multiple TV shows, but this is his feature debut.
Answer: It's never explained. This is what's known in the film industry as a McGuffin, a term popularised by Alfred Hitchcock, which is an item that's crucial to the plot, but the exact nature of which is never revealed because it actually doesn't matter in the slightest.
Tailkinker ★