Yeah, I know this is an old thread, but I thought I'd dig it up since I just saw the movie a couple days ago for the first time and the friend who brought it over explained to us what took him about 16 times of watching and debating it on forums.
There's really 4 timelines in the story - you see the very end of the first (?), the 2nd in its entirity, and the beginning of the 3rd, the 4th materializes as consequence of the 2nd and 3rd. Pretty much its implied that the first timeline was when Donny went that whole month, no plain engine fell, and toward the end of the month for whatever reason Donny ended up shooting Frank in the eye at Grandma Death's house - maybe he just broke in with some people or whatever, maybe the girl he was dating got hit by the car that time too, hard to say. At the same time as Frank died, the wormhole was there and supposedly wormholes could only send emotion and some degree of projection but not physical presence. The one detail they hadn't figured out was who ripped the engine off the plane - Donny or Frank. Regardless it was Frank who got him to leave the house in the second round so that he wouldn't get hit (not sure why), but at the same time its like he was trying to set Donny up, possibly land him in jail or whatever (or maybe in the case of Cunninham settle old scores), so that he wouldn't be in the same spot at the same time to kill him - didn't work. He did also say there were a whole bunch of alternative realities left opened by whether or not he made the choices to a). flood the school, b). burn down Cunningham's house, or c). still shoot Frank. By the end of the second timeline though Donny had figured it out, realized that none of these things would have happened if he got hit by the airplain engine, so he sent a funny thought with the plain engine rather than the thought that Frank had sent with it and somehow Frank's thought got diverted to the psychologist instead. The 4th timeline and the timelines thereafter in the loop were everything was as normal - no plain engine, no one dies on the plane, etc. etc. My friend I think explained it a lot better, I may have forgotten a few things he mentioned, but that's pretty much the gist of it.
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The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.