SPOILERS FOR RECENT CHAPTERS IN FUTURE ARC!
Many people have been having trouble getting their head around this concept, so this is my humble efforts in trying to make sense of it.
The concepts and whole story are laid out in chapters 252 and 253.
1. Parallel worlds.
– There are an innumerable set of parallel dimensions, or timelines, each of them occurring side by side to each other. In each of them, for the purposes of this story, there’s a Reborn, a Tsuna, an Irie Shouichi, a Byakuran, etc. However, the circumstances they are in, and the interactions of these people, are all different.
2. Branching futures.
– The parallel dimensions or timelines branch out, depending on the choices people make. Reborn gives the example of himself and Tsuna:
A. In the timeline we know, Tsuna has the chance to meet Reborn. There are two outcomes of this: 1. Tsuna meets Reborn and Tsuna becomes 10th Vongola boss, 2. Tsuna does not meet Reborn and continues to be a whiny spineless adult.
B. These two outcomes result in two futures, and thus they create two timelines, in other words two parallel worlds.
C. These two timelines are independent of each other, and neither timeline knows about the other.
D. Each timeline is further divided, as other decisions are made with their own outcomes. So, from two timelines, we get four more branching timelines, four parallel worlds. These four timelines create eight more. And so on and so forth, infinitely.
3. NORMALLY these branching futures are independent of each other.
– Each branching timeline is independent of each other, and no timeline knows about or has contact with the others.
– But each timeline got its own expected set of people. So each timeline has a Reborn (maybe in some he’s Arcobaleno, in others he’s not), a Tsuna, a Shouichi, a Byakuran. Just in different situations. None of these people know about their selves in other timelines.
4. Byakuran is a special case.
– Because of certain events and mistakes (which I shall leave you to read), Byakuran is synchronized with all his selves in all these parallel worlds. In other words:
A. Byakuran knows about ALL the parallel worlds.
B. Byakuran has access to ALL his selves in all these parallel worlds.
C. Byakuran can access what his other selves learned in other situations and timelines, for his use in the timeline he is on.
D. Of course Byakuran used it to his advantage.
5. Byakuran can be defeated in only one specific timeline.
– Majority of the existing timelines were dominated then destroyed by Byakuran.
– There is just one timeline, one future where all factors and people fell into correct place, where Byakuran is defeated. That timeline is the one we’re all reading.
– Why? This is the only timeline where Shouichi met Tsuna. Shouichi fixed Tsuna’s bicycle in third year junior high.
– (Remember that Shouichi met Lambo with the ten-year bazooka independently of Tsuna and the rest of Vongola.)
– Thus, what Shouichi knows falls into place. What he learns about Vongola, and what he will contribute to Vongola also falls into place. What Tsuna does as the 10th Vongola boss falls into place.
– Why not deal with Byakuran in the past? Because all these factors are not in place yet.
*The concept is named after a group of orchids (class Phalaenopsis) which create new branches when you divide original branches.
So far that’s what we know. I’ll TRY to answer any other questions in replies, but I’m not sure how well I will manage.