On term that I use a lot is “Illusionism”. The standard definition of “Illusionism” is GM’s offering player’s a game choice which appears to matter but which does not in fact matter because the GM uses his role as secret keeper and as narrator of the fiction to hide the player’s lack of agency from them.
I can take this definition, as I see the big part is the offering. This is a big problem a lot of DM do is that they keep offering the players choices, even more so when they don't want too. So the answer is simple enough: If the DM wants anything to happen or not happen, don't add in any player choice. Simply keep things separate from the players choice.
“Schrodinger’s Map” is one example of illusionism.
But here you cross over with Improv. A lot of DMs don't write down every tiny detail on a map. They just make up stuff as the players pick a direction. Your making it sound like Improv is Illusionism and is wrong.
Similarly, you can use something like “Schrodinger’s Stat Block”
Again, this steps on the toes of Improv. Not every DM can make a stat block in seconds. And even talking a bit of time, like ten minutes, it can be hard to fill in all the details, and more so fill in all those details perfectly. Your making it sound like Improv is Illusionism and is wrong.
Most commonly in my own game I use “Schrondinger’s Time Warps”. A lot of times in my game I know that certain events are supposed to happen at some point, but since games often proceed in ways I can’t plan for, and since I don’t always have accurate distances between X and Y or precise locations of everything that is happening off stage, I have a lot of freedom as a GM to determine when things happen and how long it takes for something to happen.
This might be going too far. There are only three choices here: the DM decides, the DM rolls at random, or the DM makes up a massive timeline. Most of the time it is simply impossible for a DM to make such a time line. There are a couple ways to do things at random, but a lot of DMs don't like giving up control of the game to the dice.
And, there is no fix for the problem that even if the DM had set the round the next force arives a year before the game or just rolls some dice at random and it happens at a time the players don't like, they will still complain.
The trouble you have in these situations and the reason the temptation to Illusionism is so insidious is that you can’t ever actually be certain you are being fair and unbiased.
I would say it is hard, but it's not impossible.
If you are making any choice as GM at run time during a game, there is always the chance that even unconsciously you are making choices that favor your desired outcome.
The bit of the flaw here is the DM simply can not have a desired outcome.
But in my experience players aren’t stupid. I play with a lot of really smart guys, and sooner or later if I’m fudging everything they are going to figure that out, and for some of them that’s going to be like winning a chess game against a superior player and then finding out he through the game to bolster your self-esteem.
I'm not sure most players would catch on. Really there are only a handfull of the hard core players that are always actively looking for things the DM is doing that they can complain about.
I don't think it's possible to entirely avoid illusionism, as to do so would imply that you had accurate a priori knowledge of the entire fictional world. But I do think that you should try to avoid illusionism and limit conscious illusionism motivated by "what you the GM think should happen" to a minimum. I believe high illusionism where the fiction exists solely in response to player actions is incompatible with both "step on up" and "play to find out what happens" play.
There is the Third Option: Randomess. If the DM leaves nearly everything up to the dice, you need no illusions.
Suppose you had a Gygaxian tactical scenario where you had some complex terrain and waves of humanoids with various tactics they'd employ.
How would you feel about scaling the number of "orcs" that attack the party to how well the party is doing? If they don't use good tactics then very few orcs attack, but if the party is using good tactics like employing cover, creating obstacles, working together in a formation, prioritizing targets, etc. then more and more orcs attack to keep it exciting?
This is a classic and very old complaint of "when" can a DM change anything. Like the day before the game the DM makes an Orc Squad. Then the next day of the game, once the combat starts, the DM sees that they made a mistake, or an error, or forgot something or such. So can they change it? Many would say "no" . Many would say the DM just has to "live with it".
Though overall, this is not a big point. The DM can just make the NEXT group of orcs tough.
Let's be clear here: illusionism is fundamentally a force play — it's forcing a GM-desired outcome in spite of a mechanism for choosing between multiple possible outcomes (be that mechanism a player decision, a random generator, a subsystem of the game's rules, or whatever), while pretending to defer to that mechanism for the sake of hiding from the players that the GM is the one making the decision.
The answer here is just for the DM to make things ahead of time that have a good chance of directing something, but not forcing it.
And even better: simply not giving the players a choice. No choice, No Illusionisum.