Well basically, you have the Tangant Universe and the Primary Universe. The Tangant Universe (in which Frank is present which isn't Frank himself really, but himself as a munipulated dead person) started right before an artifact (which is made of metal ie the jet engine) crashed into Donnie's room while he was asleep on the golf course.
The Tangant Universe consists of two primary factors; the Living Receiver (Donnie Darko) and the munipulated dead (Frank and Gretchen). Frank uses portals to go through time to use Donnie thus giving him "powers" to use to help stop the Tangant Universe before it collides with the Primary Universe thus negating all of existance if open for one month (or 28 days).
The Tangant Universe's element is water (the liquid that emits from a person's body that leads them from point A to point B, as you might remember in the movie) which is clearly shown when Donnie floods the school and once again in another form (as fire), leading Donnie to burn down the motivational speaker's house creating a chain of events that leads to the party after Donnie's mom is asked to take the girls to L.A.
Gretchen, on the other hand isn't given super natural powers but is used in the chain of events to aide Donnie in succeeding in stopping the end of the world. She sits next to him in English class, later becomes his girlfriend and later shows up at the party (which would have never of occured if prior events (the fire in the house) didn't take place). The letter to Grandma Death caused the living Frank to swerve to miss and run over Gretchen, as it was her role. The "Cellar door" was used in the events to have them go into Grandma Death's cellar where the bullies threatened their lives, creating Gretchen to lay on the ground, choking for air (when Frank ran her over and killed her). This caused Donnie to realize he had to kill Frank to make other events occur in the future. He drove Gretchen to a hill as he watched his mom's "redeye" flight home to get sucked into the portal, disguised as a tornado, with the engine of the jet getting sucked through time.
Then, you see Donnie in his room, laughing and realizing that death isn't as bad as it may seem. Him and himself only knows what had happened (since he is the Living Receiver) while he is back into the Primary Universe, 28 days prior. His sacrifice of death caused the Tangant to stop before the deadline and sent the engine back which in turn, caused it to not hit again and not kill people.
So basically, to sum it up, his powers are used to close the black hole (the warp through the fabric of space and time) with the aide of Frank, Gretchen and even his teachers (the Munipulated Living). The reason why he is chosen is because he would have died from the engine, so he was the closest person to this wormhole. But, since he didn't die, an event never occured which was supposed to occur in the first place.
Since Donnie died, the loop of life was opened, closing the portal and causing certain things to happen and other things not to happen; things that may have happened after he died. The Tangant Universe's lifecycle is one month and it only moves in a straight line. If it is open for one month, when it dies, since it doesn't work in a loop of life, but only straight forward, it will destroy the Earth with the end of it's existance.
I don't know, does it make sense? It was a bitch to try to word it so it could be understandable plus, I lack sleep as of late.
