The Sliver Model proposes that reality consists of a set - a subset of the entire set of slivers housed in the gallery - of timeless instants. There can be multiple realities, therefore. The slivers within each such subset link up sequentially with each other (the set has order). They are 'flipped' through, according to principles of relativity, gravity, thermodynamics and the like.
Using the tic-tac-toe analogy, reality may be thought of as a game. The game is played according to rules. In tic-tac-toe the players take turns. They mark one grid position at a time, starting with X. They continue until the grid is full, or until one of the players has three in a line. The universe that we inhabit is a game that follows its own rules.
Our universe is constructed according to various physical constants: the speed of light, the gravitational constant and so on, as well as relationships alluded to by equations such as e = mc squared. Therefore, if two adjacent slivers (differing by one pixel) are compared, and the difference requires that a photon move faster than the speed of light, then those two slivers may not exist as part of the same universe. They can not be in that same subset (as prescribed by the rules of the game) of slivers. It must be possible to 'get from' one sliver to another within the rules for those slivers to be a part of the same universe.
To understand this better, let's consider the concepts: proximity and directionality.
Proximity involves what we think of as time. The number of discrete steps it would take to get from one sliver to another, following the rules, is a measure of how far they lie apart in time. For example, to go from a tic-tac-toe grid with one X and one O to a game position where there are three of each would involve 4 moves.
Directionality involves what we perceive as time's flow from the future to the past. In the previous tic-tac-toe example, the first position of play can only precede the second. It cannot follow it. In that game, and in that universe, according to the rules, you can't go backward in time. Eggs may not be unscrambled.
According to whichever rules you focus upon, corresponding universes arise through self-selection. That subset of slivers projects itself from within the superset of all possible slivers. That's what's happening (what happening consists of).