If a mage enters Twilight and remains in the physical world, their form becomes invulnerable for the duration (AM5 p 89).
If a mage succeeds in avoiding Twilight they spend a Diameter (two minutes) bringing their magic under control (AM p 88).
The question is, during that two minutes avoiding Twilight is the mage vulnerable?
Can they take any other actions during that duration to defend themselves.
If they are attacked or interrupted or prevented from controlling their magic, do they slip into Twilight?
The statement in the rules that a magus might well choose to enter twilight without trying to control it if they were in a situation like combat seems to indicate that resisting twilight probably takes full concentration and precludes doing things like dodging or running away.
My first instinct is to allow some minor actions with a concentration roll, I'd use the same difficulty as doing something while casting another spell (15) with failure leading to automatic failure of the twilight control.
My personal read on this is that you are mortal until you are in twilight. If your head gets cut off prior to you entering it, well, you are cut off from your head with all the advantages and disadvantages that comes with it.
If your head were cut off before the point you would have entered Twilight, you're dead.
Now, if the scenario is such that the player knows his character is going to buy the farm, if he doesn't stop fihgting twilight and instead chooses to go into Twilight, sure, I'll do that, and his Twilight begins moments before he is struck with the sword that would have separated his head from the rest of his body.
Is this in reference to my manipulating the story to save a magus, or at least delay his death? I'm not violating the limit of Time, so much as I'm applying retroactive continuity.
This would also be a series of very specific events, and whether I apply it would depend on any number of factors, and would probably involve a discussion with the troupe, to boot.
I have little prohibition of killing a grog.
A companion I have a little bit more.
A magus, though, represents a lot of work on the part of the player. I want to make sure that they are ok with the exit. Does it serve the overall narrative of the saga, the covenant and to a lesser extent the character. The magus going into Twilight may not save their bacon. It might make things worse when they come out and find that they've been utterly surrounded and the opponents are just waiting for the magus to become vulnerable again. In short killing a magus or a companion is a negotiation as to whether the character's story is over, and I really hate to see the arbitrary nature of die rolls to be the sole determinant of whether a magus dies. Now all of that goes only so far; if one makes a lot of foolhardy moves and generally plays their character without concern regarding his life, my willingness to alter the narrative of the story is going to evaporate proportionally. Oh, and this isn't a get out of jail free pass. There will be consequences to adjusting the narrative, above and beyond the Twilight experience. Going out in a blaze of glory is fun. Going out because you botched your defense roll, and the bandit slit your throat is not.
It's a do over, a take backside. It let the player take the riskier choice without fear of suffering the consequences because we retroactively change the decision to avoid the effects. If you want to have a character who avoids death easily in most situations, take the Death Prophecy virtue.
Except they get the choice twice First when they are going into twilight and choose to try and control it and then a do-over when they are going to die and the choose that no, they didn't really want to have controlled it, they want to not die. There is a major advantage for that, called Death Prophecy, which you are apparently giving all of your mages for free.
Well, no, because if they do stuff expecting to get saved, they'll run out of chances, and they die. No warning, no, you must complete your destiny, no nothing.
Instead, I'm suggesting that someone who has put a lot of time and effort into a character and the saga has some degree of control in determining how their character goes out, and I'm not going to leave it entirely up to a die roll, unless the player is OK with that. It's not a death prophecy, because they can die at any time, some of that is up to the player, based on how he's played the character and a lot of it is up to me, again based on how he's played the character. Further, I'm not bound by the Death Prophecy, in that I don't have to foreshadow stuff, and generally speaking characters cannot get away with insane risks.
In terms of the original question posted, it is definitely not RAW.
In fact I believe the RAW intent is closer to the idea that you would need a concentration roll to dodge than that you could retroactively decide that you didn't try and control the twilight in order to save your hide.
We have used twilight to save the hide of our magi before. I even saw once a magus casting HIMSELF into twilight to save his hide there. It did not break the game at all. It is just a feature and caused a cool adventure in the magic realm for us. Being hit by twilight is a potent attack vs a hermetic Magus in the game, but fairly Hermetic-specific. Other traditions ar enot as vulnerable to it, and so its usefulness to take down other critters is limited.
So my thoughts are that:
yes, you need Concentration rolls (as per vasting a second spell in difficulty) to do something else.
yes, if you fail the Concentration roll you cannot control your magic. You can either send the magus into twilight automatically OR play it as a magical botch. Basically your are plugged into the magic realm while trying to control the effect, and failing the concentration causes a plug spark.
I would agree with these, it's the idea that since you have now taken a lethal blow you can choose to have entered twilight 7 seconds ago avoiding the blow that made you decide to embrace twilight that to me violates the spirit, the RAW, and the limit of time.
From my PoV it would be reasonable to choose to stop resisting twilight when the attack is declared, but not after it's been rolled. Just with penalties to your twilight understanding (or even an automatic fail).
No retroactive choice, but it only takes a split second to choose. Once you know their attack roll it's too late.
This is just as much a retroactive choice as mine.
What happens if the twilight was resisted (successfully) last round, and your shield grog botched his defense roll, and thus bought a farm. Now you, as the magus are defenseless, and you will also buy a farm. Unless, you allow one to slip into Twilight.
My point is that magi are difficult to create and have a big impact on the overall story of the covenant. Giving them a little wiggle room to survive isn't really against the spirit of the game, especially a story telling game. By and large, players should have a big input on how and when their characters go out. Stupid behavior trying to take advantage of this "Death Prophecy" as silveroak likes to call it isn't going to allow the character to last very long. Again, I try and not let the entirety of an arbitrary die roll be the sole determinant in how an encounter goes down. A lot of how it will unfold is dependent upon whether it is the final element in an episode, or an early scene.
Being hit by twilight is a potent attack vs a hermetic Magus in the game, but fairly Hermetic-specific. Other traditions ar enot as vulnerable to it, and so its usefulness to take down other critters is limited.