The flashbacks, when part of a "mental echo" of a highlighted corpse, allow you to change how the world is in the present. You apparently save the person from death, but also open the path for yourself. If they are not dead, then the door is opened, or a lever is pulled, or you are otherwise able to proceed. Of course things are not at all okay in the past either: something terrible has happened to the ship, and things are attacking you. Apparently, the captain of the ship is crackers too, and that doesn't bode well.
It's all rather frightening, but the frights are reduced and rendered somewhat irritating by the fact that your reaction to them is always so necessarily clumsy. Your control within the world feels gloopy and imprecise, as if you're a bit dizzy and trying to fight some treacle. Yes, you've heard this complaint a hundred times before: nice ideas, shame about the execution. This theme of "not-quite-there" runs throughout Cryostasis. It begins with a couple of instant-death situations, and escalates into puzzles and then fights that have to be repeated over and over, returning back to the checkpoint again and again, because they're just difficult and unfair.
Far worse still is the way the time-travelling system directs you. After a couple of jaunts into the past you realise there's no real game to it at all, despite that fact that you might beat on a zombie, or avoid plunging to your death. It's not problem-solving, or decision-making; it's just following the motions. You aren't deliberately changing the past to affect the future - you never think "oh and can solve this via that flashback" - you're just doing it, because that's the next sequence in the way of progress. Some of those sequences are extremely atmospheric - especially where they involve deep, cold water and hideous swimming zombies - but it might as well be a linear corridor shooter with buttons to press to open the next door. The net result is that much of the game is extremely laborious, which detracts enormously from its atmosphere of threat and mystery.
Which brings the game back to the actions-versus-brains thing: it's a game that wants to tell a story, and to commit completely to its grim, frozen, time-skipping nightmare world. That's admirable, but it does also make the entire experience feel reminiscent of point-and-click adventures of the past. In the worst of these games puzzles are not really puzzles, and the linear environments are little more than a stage on which a story - often rather awkwardly told - slowly unfolds. Except Cryostasis isn't an adventure made 3D and interactive; it also wants to be a first-person action game, and that doesn't quite work either.
Cryostasis is a brave, fascinating, often very beautiful game, but I find it impossible to recommend it - and not least because it runs like an exploded dog on most PCs. It's not quite creative enough - its environments fall into a monotony of samey rooms and bulkheads - and its combat is too clunky to be delicious. I'm certain that this is a game that some gamers will tuck into heartily, and happily. The ugly truth of the meal, however, is that's it's still frozen in the middle.