Yes, I was more interested in what what you think the particular way is that works well for your favoured sandboxing.
Having given this more thought, I'm really not sure what you're looking for here. We were talking about constraints, so I presume you are asking in what particular way is the GM constrained when running a sandbox in my favoured style, but I've already covered that to the extent that I can.
Ultimately, the GM is constrained by the need to to offer something that satisfies the players.
As mentioned previously, as far as I can tell, that is the only real constraint that
any GM operates under, other than self-restraint.
Luke Crane opted to layer additional, formal constraints underneath that top level constraint, when he wrote Burning Wheel but I, in general do not.. This does not mean I act unconstrained, because that top level constraint does always and must always exist.
I tell my players what sort of game I plan to run and I am then constrained by agreement to run that. Should I feel the players are not operating within the agreed constraints, I raise it with them. Should the players feel I am not operating within the agreed constraints, they raise it with me. If I feel I am not able, in practice, to provide the game I originally offered for some reason, I raise this.
In any of these situations, we come to an understanding and move forward. One or any of us may need to adjust our actions
or our expectations, and we do so. But there's no deep, mysterious process at play here, it's fundamentally just normal social interaction not unlike in any other group activity.
If you're talking about my processes more generally, as opposed to constraints specifically, my general preference is to develop as much of the world as possible in advance. I will sometimes work on a game for years before I run it. I am currently working on my Rolemaster Savage Frontier game in my spare time, while running something completely different. I am in the process of adapting material from the 1e Savage Frontier book, working out details of adventure sites, towns, weather etc. I will create encounter tables for various locations based on the things to be found there. I will most likely develop some key fixed events likely to occur over a number of years if nothing the PCs do interferes with those things. I will also aim to have a number of random event and random rumour tables to lean on in play as required. I will probably pre-generate a number of rumours/events to occur at random intervals, and weave them into whatever else is already going on.
I will ensure the players are provided with plenty of information about the region so they can think about their plans. I will most likely suggest a number of possible starting courses of action based on opportunities for work/adventure in the region and allow them to work out what it is they wish to do (whether it's to take up one of those opportunities or something else of their own devising). The plan is for the PCs to be running a small mercenary group, so they will most likely need to make some decisions about outfitting. Their initial plan will most likely need to account for the fact that the ability to campaign over long distances and with large numbers of troops will be difficult over the colder months -- they will need to plan ahead and have somewhere for the troops to winter.
Then, the players will confirm what it is they intend to do, and away we go. As much as possible, I want to be able to refer to existing material whenever the players decide they want to go somewhere or do something.
Unlike Rob, I'm not up for the players deciding to pack up and head off somewhere completely different. I will expect them to actually go adventuring in the Savage Frontier region, not run a cake stall in Waterdeep or immediately board ship for Calimshar (sp?) but, within that fairly broad scope, they're welcome to do as they wish. I have not the faintest clue what they'll end up getting up to, how the world will change, or all that jazz, and finding out is where I will have a lot of my fun.
Again, a lot of that is probably pretty vague. I'm not interested in a deep analysis of my processes and I'm unlikely to elucidate much beyond what I've already written. Just writing up this much was something I've done as a gesture of goodwill. If you find it useful, great, if not, sorry I was unable to give you whatever it is you're after.
I will note that I am uninterested in any line of followup questioning that compares my process to any other and asks me to consider which is better in any kind of objective sense or questions why I don't do use some other method. My process works for me. If someone else's process works for them, great. I'm not interested in being asked to offer further defence of what I do
or of attacking any other method.