I probably got a bit too deep too quickly with my previous filter post.
Let's look at some simpler options...
An adventurer is exploring ruins. She has a range of tools at her disposal.
In traditional games, the GM sets filters for the adventurer, then (depending on the game and the specific scenario) the player choose which of these filters they want to push their character's story through.
The GM has set an additive filter and a subtractive filter for this encounter.
If a Narraton passes through an additive filter, there is a chance its power is increased. Aspects of it's wavelength are enhanced if the filter is negotiated successfully...or nothing happens if the filter is negotiated unsuccessfully.
If a Narraton passes through a subtractive filter, there is a chance its power is decreased. Nothing happens if the filter is negotiated successfully...or aspects of it's wavelength are diminished if the filter is negotiated unsuccessfully.
Example Subtractive Filter 1: Punji Trap
The GM basically springs this one on the character. This may occur when the character moves over a specific part of the map, or it may occur at the whim of the GM.
Success: The adventurer spots the punji trap and makes their way around it.
Failure: The Adventurer falls into the punji trap and suffers damage. This impedes her ability to go further in her quest (her health is reduced), and in turn her overall drive in the story is set back.
Example Additive Filter 1: Hidden Ruby
The GM can invoke this filter without conscious effort on the part of the player as well. Again, this may occurs when the character passes the ruby's hiding place on the map, or it may happen at the whim of the GM.
Success: The adventurer finds the ruby. This may prove to be a valuable tool for negotiating an obstacle later in the game, or it may be a trade item for when she returns to town. Either way, her ability to traverse future filters in enhanced through this new addition.
Some of the more recent games allow players to set their filters. Perhaps the adventurer goes out of her way to look for antiquities while she is in the chamber with the ruby, Thus she sets a new additive filter for herself (with a success threshold determined by the GM or by some mechanism within the rules).
At a later point, the adventurer may declare a new challenge for herself by playing on one of her weaknesses, or choosing to explore something about her inner self (like activating a "key" in The Shadow of Yesterday), if she fails then something bad happens, but if she passes, she might earn experience points for facing that demon within.
There is nothing to stop a filter being both additive and subtractive at the same time (gain a bonus if you pass, suffer a penalty if you fail).
Otherkind dice really reflect this idea well. In most instances of their use, you get at least two dice, one is assigned a potential positive outcome for the situation (an additive filter). Another is assigned a potential negative outcome for the situation (a subtractive filter). These filters are passed simultaneously, and the narraton emerges on the other side with varying states of change.
yes +ve / yes -ve
yes +ve / no -ve
no +ve / yes -ve
no +ve / no -ve
Some variants of Otherkind dice then allow extra stakes to be added into the matrix, providing additional simultaneous filters to traverse.
For every additive filter passed successfully, something is changed within the story to benefit the character. For every subtractive filter that is not passed successfully, something changes with the story to the detriment of the character.
With this in mind, the traditional method of scenario design becomes pretty simple as well.
1. Create a scene.
2. Apply one or more additive filters to the scene (with relevant success thresholds).
3. Apply one or more subtractive filters to the scene (with relevant success thresholds).
4. Start again (but this time increase the thresholds a bit).
With a string of scenes (and filters within those scenes), we can see how a narraton should pass through the entire scenario.
A character is expected to pick up the magic sword along the way because they'll need it to slay the monster at the end. The act of picking up the sword within the story is reflected by an increased ability to fight within the game mechanisms. Without the sword at the end, the threshold of confronting the monster might be too high to pass, with the sword it becomes a real possibility.
A character succumbs to too many traps along the way and the more injured they become, the harder it is to pass subsequent filter thresholds. Eventually one will stop the character's story permanently (unless they take a rest break...which in effect is an additive filter, providing refreshment to the health part of the character's spectrum, allowing new spells to be memorised, or equipment to be fixed).
At a larger scale like campaign design, character progression sees a Narraton intensify as the character becomes more capable. At low levels they are hindered by weak filters, while at high levels they simply pass through these filters like x-rays through paper. Every additive filter providing a new tool to overcome story adversity, every subtractive filter adding depth to a character by revealing their flaws.
I'm starting to rant again...so I'll stop here.