But for the moment, I'm fascinated by the concept of "Flow" (especially as seen in a current thread at Story-Games).
There are numerous ways to look at the concept of "Flow", but I'm going to restrict myself to two.
Flow from the perspective of the characters within the game (which I will refer to as Game Flow).
Flow from the perspective of the players experiencing the game (which I will refer to as Metagame Flow).
Flow is described fairly well in the thread, so I won't go into too much detail here. Suffice to say, it seems to be very similar to the concept of Zen "no mind". Flow is the situation a person finds themself in when they become so absorbed in an activity that they start to lose a sense of themselves. Time starts to lose it's meaning, the action is all that's important, decisions take on an instinctive value.
Flow seems to require a wide variety of effects to combine at a single moment. It almosts seems to be an transcendental state of being, requiring enough skill to understand what is happening, enough awareness to fully sense the surroundings, and enough willpower to overcome possible distractions.
Flow manifests many ways. The swordplay of an expert duelist, the mastery of an expert blacksmith, the unerring eye for detail possessed by a master tracker, the union of man and machine in a high performance race-car, any activity can be subjected to flow.
A crude method of achieving flow within a game could be the phenomenon known as a "critical hit", or an "exceptional success". This is when a character within the game is the recipient of a naturally high die roll, or accumulates an unnaturally high degree of success through a highly lucky set of circumstances. But most games simply leave it at this.
Flow should be something more special, it should reflect the way a character truly transcends themselves, almost becoming one with their tools, their surroundings and the task at hand. Descriptions should become more vibrant, more dramatic, more intense. In a game without magic, flow should seem magical. In a game with magic, it will probably be even more mysterious an mystical.
Flow probably shouldn't happen very often, but when it does, it will leave a memorable effect. When playing a game such as D&D with distinct levels, experiencing flow might be a time when a character transcends from one degree of power to the next (perhaps characters specifically have to wait until they achieve a moment of Flow before they may progress in the game). In a game with skill improvement through expenditure of points, a moment of flow provides an appropriate justification to spend those points.
These are moments that change a person, and they remain a distinct part of a character's history.
Some character may try to achieve flow more regularly, but the exact method by which flow is achieved should remain mysterious.
Some players experience something akin to Flow when they forget that they are playing a game and they really think from the perspective of the character within the game world. Others experience it when they forget the mechanisms and the rules of the system and work toward the stories emotional or dramatic goals.
But all too often, roleplaying games are filled with distractions, and distractions are anathema to flow. Some people find visual ciues such as props or maps to aid their chances of achieving flow, others find these pieces of ephemera to be distractions. Some enjoy elaborate descriptions, others find these descriptions to simply be long-winded.
I can't write a catch-all method to achieve flow wthin a game, but I believe it's something we all try to attain.
A good GM will know the tyes of things to bring out the best potential for flow within their games. They'll play to the strengths of the players and their characters, and try to keep player tension to a minimum while character tension may be raging out of control.
Many people seem to agree that Flow seems easier to achieve in a live action roleplaying game, this may be due to the reduced illusiory barrier between characters and players. It's also interesting that the discussion of flow has passed through the context of computer games on the Story-Games thread. Is this something to consider about the difference between computer games and table-top pen and paper games? Computer games resolve the rules in the background, traditional pen and paper roleplaying games can be distracting with their rules. Many modern independent games walk a line between these extremes, trying to achieve specific styles of play with their streamlined sets of rules.
I haven't decided whether these modern independent games have honed in on the potential for flow, or if they've simply eliminated the number of ways that an individual might achieve Flow within a game.
Like most things, I guess that this question will be answered in different ways by different people.