So, the DM frames the scenario - we'll keep beating the horse of crossing the desert. The players, if they want to engage in that scene, then begin filling in details - how do they do it, like you said, buying camels and making a caravan, whatever. OTOH, if one of the players just isn't really feeling the love, generally speaking, that player is going to do something to shortcut that scene in some way to get to the next scene - thus the giant centipede.
That's just so vague I have no clue what you are talking about.
The DM simply accepts that shortcut and moves on to the next scene.
How do you recognize these short cuts? How does a player mark that he wants the scene reframed? Is there ever negotiation on that point? How would the DM counter-offer? How would a player go about counter offering? How does the table resolve issues like a feeling that one player is grabbing too much spotlight?
However, since I feel, and I think the group agrees, that nothing in the game world actually exists until such time as the PC's engage with it in some way...
That's just a MASSIVE difference. Talk about validating Celebrim's Second Law. I have the exact opposite stance. Everything in the game world exists independently of the PC's or thier actions. It's there. Even if I haven't previously written it down, if you ask me to think about it I can churn out a detailed description of just about anything.
How do you mean reframing?
Ok, so first let's define 'scene framing'. Scene framing is what the story teller does when he describes what the characters are currently seeing and experiencing to the players so that they can share in the imagined space. It's when you give the actors there scene. "Ok, you've just remet after not seeing each other for a long time. You are in a city called Amalteen, which is an important port town and you having breakfast on the patio of this tavern which is right on the harbor."
Now the players can start interparty role play based on this scene. Or they can query for more information about the scene like, "What's for breakfast?" To which I can respond, "It's late winter and so fresh food is pretty scarce. Most people are having cold curds, toast with butter, and salted fish. Some people have oranges, probably imported in from Irendi, a nation to the south Amalteen is allied with." Or they can start to interact with the environment based on the information, "I'd like to throw some bits of toast to the sea gulls." Ect.
To make it even more concrete, this is how I scene framed the entire campaign:
"It’s is the third hour of the day on the 12th day of the second month. The morning sun shines thin but clear in the chilly winter air. In the harbor district of Amalteen the life of the city is fully underway. It is Wallsday, and many skilled craftsman have shuttered their shops, but the commerce of the harbor continues unabated. Shouting above the barking dogs and screeching sea gulls, the fish mongers hawk their wares. Several ships have come into port on the morning tide. The stevedores turn capstans to power the cargo cranes while singing out there work songs. A wrinkled and grey headed hill giant sings loudly and off key as he works along side. All this is occasionally drowned out by the bellowing of mastodons dragging their carts and sleds. On one of the quays, a particularly vicious looking crew of buccaneers is beginning to disembark. Goblins, Orine, and tattooed humans from barbarian lands spill out on to the dock, drawing wary looks from the well-dressed merchants there to receive cargo and news from foreign lands. It takes no skilled observer to see that the ship is in the service of the Queen of Irendi, as all the officers are tall, red-headed, and green eyed concheer, some of which may have noble titles to go along with their true names in their homeland. One particular barbarian, a particularly tall and ruddy skinned mokoeen stands out from the rest, as even his fellow crewmates seem to fear him and give him a slight berth as he stands at the base of the gangplank taking in the city."
So that's 'scene framing' and it should be pretty darn clear that in your average D&D campaign, players never talk like that or read prepared descriptions like that. But equally it should be a very familiar technique, because every time you walk into a 30'x20' room the DM has to tell you its dimensions and what it looks like and whether there is a jabberwocky about to eat you. D&D tends to rely mostly on continious scene framing without explicitly doing it except at the beginning of adventures or sessions, but sometimes you have more explicit scene framing in long journey's when encounters are or interesting locations are infrequent.
So, what is scene reframing? Well, it's not for example starting a fight in a tavern. It's not conjuring a few fiends from the lower planes and siccing them on the commoners walking along the port. It's not deciding to teleport away, though that will probably immediately lead to the need for new scene framing. To propose that any of the players actions caused scene reframing would be to suppose that there was some fixed purpose to the scene, and that by changing the purpose you've changed the scenes. But purposes and goals are entirely the domain of players. As a DM I'm not wedded to any particular outcome of the scene. Whatever the actors decide to do with the scene is there business. Hopefully its entertaining. But it can't really change the scene. Now, an actor could say in effect, "I want to walk off stage left and see what's there.", and now I need to frame a new scene. But again, the act of declaring your intention to walk from here to there is separate and distinct from the game master now narrating what there is like. The intention to walk from here to there is merely a proposition. The narration of what you find is the scene frame.
So back to scene reframing. Scene reframing is when you renarrate the scene in responce to new important information presenting itself. No, again in D&D, scene framing tends to be continious. Players may occasionally get into an intraparty mode where they just go with each others flow for a long period, but some groups never do that and most groups stick to the party caller/DM responce model of play described in the 1e DMG as default. So scenes tend to subtly be reframed over time without any real need to mark this or have special terminology for it in my opinion. If you were going to call it something from cinema, I'd call it a 'tracking shot'. The camera moves continiously with the PC's. But sometimes in D&D you really do have a big scene refrain. Here's how I reframed the current campaign about 15 minutes into the first sesson of the game (skipping over some smaller but important reframing that actually occured in the session for dramatic effect):
"Gangplanks start falling off the anchored ships and clattering onto the piers. Small boats suddenly disappear from the edge of the quays. Several of the big tall-masted sailing vessels mysteriously start to slowly keel over, there masts and rigging tracing a deceptively slow arc through the air like falling trees. Sailors begin to panic, some jumping off of their ships and a few lashing themselves to the desks with ropes. The stevedores have ceased their labor, leaving behind primarily the sound of agitated sea gulls. Much of the crowd is now frozen, staring out to see and pointing, though a few have oddly started jumping over the sea wall."
Something important is happening. The scene has changed from a normal pleasant though not quite tranquil morning in Amalteen harbor, to something more mysterious and ominous. Now imagine for a second you are a player in a typical game, and you just decide to say to the group the above scene reframing peice. The other players would look at you like you are crazy. The DM would be like, "What? Wait a minute?" Heck, I reframed that scene and my PLAYERS were like, "What?". My players went into shock. Imagine if a player took on that narrative authority. How would you adjudicate a player doing scene reframing like that? You've claimed that players have that inherent authority to scene frame, and I'm struggling to grasp how that works.
So there is to my mind a really big difference between a player saying, "I conjure a giant centipede.", which is a proposition, and a player saying, "Ok, I conjure a giant centipede. It has a 40' climb speed and can climb over any obstacle so we can just go in a straight line and I can forced march it until it dies. I figure thats about 60 miles a day, so in three days we get to the Witch Kings tomb on the other side of the desert." The second is the classic outcome as proposition gambit by a player, and the novice DM falls into its trap by taking it on its own terms and trying to argue with the rules interpretation. The skilled power gamer smiles and argues and argues until he gets a satisfactory rules explanation, and then the DM realizes he's just agreed to reframe the scene to the Lich King's tomb and he's wondering what just went wrong. What went wrong is that everything that the player said after, "I conjure a giant centipede" was not only irrelevant but broke the social contract. Players get to offer propositions. They don't get to offer outcomes or scene reframing.
I would say that any time the players (or the DM for that matter) changes the parameters of the situation significantly enough that the scenerio is now very different, that's scene reframing. At least, that's how I've always understood the term. So, yes, I do consider Greenfield's example to be the players reframing the scene. The original scene was a dangerous climb down the cliff, possibly falling, roping together, using various skills, maybe even an attack by some flying critter half way down. The reframed scene is a 30 second narration by the players of what amounts to essentially an elevator ride.
First of all, the original scene wasn't a climb down a cliff. That's something undertaken by the actor. The original scene was a road, a cliff, and three watchposts. There is no climbing in the scene until the actor acts within it. The actor deciding to climb, or fly, or walk down the road isn't seen reframing. The scenario hasn't changed. It's still a road, a cliff, and three watchposts. There is no such thing in an RPG as a framing a scene as a dangerous climb down a cliff. The DM isn't writing a script for crying out loud. The players can just come to the cliff, take one look at it, and say, "I'm not going that's way. Let's go back to town and get a beer." And that doesn't reframe the scene either. That is the scene.
But careful. Where do players get the right to a 30 second narration of what happens, and how it works, and what the outcome is? Players can only describe their actions. Now if it is a good plan, and there are no factors they weren't aware of, and nothing happens to interupt the plan, probably they'll just get a series of affirmations from the game master validating the plan so far and eventually they'll probably need the GM to scene frame. But they can't just say, "Ok, I arrive at the gate.", just because they think its a good plan. If the players can just say, "Ok, we accomplish what we want to accomplish without hassle.", you don't need a DM.