It's difficult to explain the sense of confusion one can have even after watching a movie the second time round. Even with this version being the directors cut, I'm still a little lost on the necessity to make it as ambiguous as it was.

From what I gather, the film splits to a seperate timeline when the jet engine collapses Donnies room. And Frank is a sort of meta spirit that is trying to correct the timeline and Donnie is the receptacle for Frank to fix it. And when the timeline runs the way Frank needs it to, it collapses. The jet engine is from the seperate timeline and is co-existing so it splits and Donnie needs to die for the film to correct the timelines...I'm still working through the film here. I'll probably edit to add more here but it's majorly fucked my head.

I commend it on Mad World, best scene in the film. The montage and the soundtrack really packs an emotional punch.