I originally taught this with note cards, but have found you can also do this in two columns on a piece of paper. Designate one column (or card) 'A' and the other 'B'. (Or some cards 'A' and some 'B') Put them aside.
Now, the theory - for creating shorter (and slightly more intensive) direct experiences, there is a basic mechanic at work. It can be expressed and explained in three variations. I will detail them below.
Note: All of these can situationally operate 'backwards' as B:A B/A and B-A
A / B
Here we have two tracks of action, an A and B, and they work oppositionally. Time is not necessarily a factor here, but this is best expressed as 'nearly simultaneous'. Conceptually, this means that while A acts, so does B. While the good guys act, so do the bad guys. Their agendas need not mesh, although at some point, they should intersect (at plot climax ideally). This setup works best in larger scale campaigns, where players (often A) can operate under a wide variety of designs and are only vaguely aware of B's actions. (If that is the case, this is called a 'Blind A' or 'Blind B' scenario). Taken a step further, you may even withhold the existence of the one side from the other, although that tends to rob the players of the satisfaction of knowing the stakes they face and therefore projecting possible reward/praise.
Note: The majority of game experiences fall under 'Blind A' conditions, as the party operates mostly unaware of what their foes are directly doing (even though they might know about the goals).
This is more a traditional cause-and-effect situation, and is stated as "If A, therefore B" In this case, A and B need not be opposing sides, they could be action and consequence, move and counter-move, or any like-bonded pair. The majority of two player board games operate under this theory. (If he moves his pawn, I will move my knight -- if the rogue checks for traps, I will guard the door, etc) In multiplayer contexts, this serves a lot masters - individual characters follow cause-and-effect, players have an expectation of having their actions 'A' have in-world consequences 'B', and NPCs assume that 'B' is the reward for their plot. In this model, the tension surrounding "what will happen next" serves the desire to move things forward, as players will naturally want to know "what happens next".
This is the most open-ended and often most subtle expression of the structure, and is stated as 'A to B'. This is not a direct relationship (that's A:B), this is more about the option of the path from one item to the next. In a linear context, this is going from a quest giver to a location, or moving from one room to the next. What is not stated here is 'C', which is often defined as a larger goal (clear out the dungeon, win the election, do my errands, etc) and that 'A' and 'B' are items on a to-do list or instructional template that lead towards C's realization.
Now, let's see how they all blend together. Let's assume you have two players (the actual number doesn't matter), and that you're starting them in a room with a body.
1. Two players are in a room, with a body. (It is natural that they will want to explore/examine both the room and the body, although more likely the body, as they will assume it possesses more potential) [A-B]
2. Two players will speculate as to who they are and why they are here [A:B]
3. While this is occurring, the world exists OUTSIDE this room [A/B]
4. The players will discover some plot elements (they have to, else the game stalls out) [B:A]
5. The players must cooperate to leave the room [A-B]
Ideally yes, the whole game should blend together these elements on a moment-by-moment scale, and yes, in theory you should be able to chart the 'beats' (the scenes, the 'things') by this shorthand.
Now as an overall game experience, we can see a larger A and B relationship. In two-session play, the first session is 'A', and the second 'B'. The variation in relationship is dependent upon the way the sessions link together, which is an expression of the desired plot (either via game or via GM). Likewise, you may designate the players 'A' and the antagonist 'B' and determine their relationship accordingly. Going further, you may see all PCs (heroes and villains) as 'A' and the world they operate in as 'B'.
Return to your paper or note cards. On 'A' list the appropriate beats or characters. Number them as well, so that you can cross reference. Do the same for 'B'. See below
1. The players are in a room with a body
2. Gary, player #1, has an Aspect of 'Don't Take Me For Granted'
3. The villain buys the election.
1. The players must cooperate to escape.
2. Engineer the situation so that Gary is compelled.
3. Therefore, he's set to become comptroller in two months.
(Using note cards is easier, since you can create note card pairs like A-1 & B-1, but side-by-side columns are effective if you have space for them)
In game design (as in storycraft of a novel) you should be able to chart out even the most open-ended experiences (hi Skyrim!) because the opportunities need not be linear, but they can be plotted (If the story/characters go to Location #1, that's an A:B, at Location #19 that's B-A).
I hope this sets some creative fires burning.
Note: I realize that I make use of the term "beats" so perhaps I'll do a second post about what those beats are....