I must also confess, like Andy, to falling asleep during "Primer." But after discussing it later with my wife, I just keep coming back to one question: Though many of the film's aspects are difficult to understand, above all, I can't seem to get a clear grasp on why there would be a second version of a time-traveling individual, namely, his present self and his past self.
Allow me to think out loud here: If I were to travel back in time in order to see my favorite toy, a Wonder Woman doll — which is no longer with me in the present — it would, in fact, be found in that visit to the past, circa 1981 or so. Now, since matter does not cease to exist, in present-day 2010, presumably, that Wonder Woman doll must surely still exist, somewhere, and in some form.
Our dumb dog chewed it up only a few days after I got that doll for Christmas, so my parents threw it away. That doll exists in some garbage dump somewhere, deep underground. And since she was made of plastic, which is basically eternal, that plastic doll lies beneath the Earth. So, there's a doll in 2010, and there's a doll in 1981. But there was always just one doll of mine. If I went back to 1981, then that doll would not exist in the dump yet, in that era, until that very same piece of matter is sent there from my parents.
If I hold the plastic dumped doll from 2010 in my hand when I travel back to 1981, what would happen then? On Christmas morning, 1981, the doll existed in its pre-dog-chewed form. But if I'm holding the dirty, futuristic version in my hand, then I suppose there might be two Wonder Woman dolls before me during that time traveler's moment. Therefore, perhaps I can understand a little better how there would be duplicate versions of the time-traveler himself.
And accordingly, then, am I to understand that if time-traveling Jason from 2010 goes back to 1981 and gets little Jason from that era, and then we both travel to 1994 to see me graduate from high school, then I would have three Jasons present, right? Maybe. Just maybe.
"Primer," obviously, is an excellent think-piece of the cinema. It does seem to be the most accurate time-travel film I've seen.