The board state is always known. Magnus is saying that at the top level, grandmasters know how to force draws in all of the lines that incorporate best-move, second-best-move, etc. That means that if you want to pull off a win, you have to incorporate unstudied lines that will force your opponent to calculate rather than draw from memory/experience.
But if you make a suboptimal move, then you are going down a path that is less analyzed and thus you are likely to have an analytical advantage.
So while the board state is well known, what your opponent has researched is not known, or partially known (since players have favorites and predispositions).
This is why playing against a maniac (seemingly random and not respecting bluffs or equity) is very difficult because if they get lucky enough times, they are able to "break the game" by getting the opponent to be extremely risk averse OR take on more risks.
I believe this is what Magnus is referring to, its that making yourself unpredictable by questionable moves and no longer playing in a way that has been taught.
For example the common strategy is to go all in with strong pairs like AA, KK but someone beats it with a totally random garbage hand (52o, 37o) and does so repeatedly, no theory can help you win against somebody who is just repeatedly lucky and brash.
This is less about playing mindtricks and bluffing, but more 'mundane'.
I.e. a decent player will know all the good mainlines of popular openings, and end up in 'comfortable' positions (among other things due to computer analysis).
The metagame is to prepare a non-garbage sideline, that your opponent is not so familiar with. Nobody at a high rating plays 'questionable' variations on purpose, in order to bluff. The resulting positions would be much too punishing.
Post-game thoughts from the players:
Inteviewer: "Let me pull you back; so you didn't understand the position, and so you still felt like you were scared to go into a piece-up situation?"
Alireza: "Yeah so, I just trusted him. (he shrugs) I just wanted to make a move.. and play a bit more you know (laughing)"
https://www.youtube.com/watch?v=xT9orNSgk5w&t=55s
Niemann: "You have to understand, when I play the move Qg3, this is a purely pyschological move."
https://www.youtube.com/watch?v=CJZuT-_kij0&t=780s
Whether or not Niemann did cheat over the board, this shows that even top players can be susceptible to this kind of bluff.
So you're taking a game where your opponent has the upper hand, and you're playing highly suboptimal moves with the intent of putting your opponent in a position where they have to pick between a rock and a hard place:
- either they have to sit quietly and knowingly take your straight-up lies, or
- they are forced to play suboptimal moves themselves to call your bluffs.
- they see through your bluff, and punish you for it
So, it is a risky endeavor, since you don't know the mind of your opponent.
And so it makes sens to only take such risks when your opponent has the upper hand, as you mention.
But then you become predictable — when you behave erratically, people know you are in your death throes, and not a real threat.
So, you have to mix some erratic behavior in when you are in a strong position, too, to keep them guessing. But then you are weakening your game overall...
And on and on it goes (:
It's like an arms race, but circular rather than one-directional.
Both games rely on disguising the real play you want to make. Slow playing the best hand in poker, or making non-standard moves in chess to throw your opponent off.
You see this in all sports/games. In basketball/football a team will run their best player away from the real play. In Jiu-Jitsu I'll fake a choke I have no intention of seriously trying in order to force someone to defend and move their arm - which is what I wanted the whole time.
It's amusing how people feel bluffing in poker is the human element, but top poker AIs bluff more frequently than most humans
Good bluffs are when your range beats your opponent's range. Which gets back to your point, which is that you can do some pretty awful things to someone when your hand is outside the range they're putting you on