The scale is 0-10000, and as it increases you have a higher chance of having certain hallucinations, these hallucinations become stronger, and once you pass certain thresholds you begin seeing newer, crazier hallucinations. For instance, with the rules he sent me, at 200 you have a 20% chance of seeing ghosts on a number of cues, and at 2000 you start occasionally seeing shoggoths when you try to sleep. The maximum is 10000.
Increasing dissociation:
Starvation and dehydration cause dissociation in the (obviously already quite unhinged) player, but not much.
Sleep deprivation and low will cause dissociation in the player, and quite a bit of it.
Some psychoactive drugs (especially alcohol) increase dissociation.
Some enemies cause auditory and/or visual hallucinations and increase dissociation.
Anti-psychotics (iloperidone, asenapine) leave you more dissociated after they wear off.
Killing other living things, to varying degrees, increases dissociation.
Decreasing dissociation:
Dissociation does not decrease during normal gameplay, but using the wait/rest/sleep feature decreases it slowly.
Anti-psychotics (iloperidone, asenapine) decrease your dissociation a massive amount temporarily, but they are counter-productive in the long term, are addictive and have nasty side effects. (If there's an elder thing standing by your bed you should probably just take the damn pills and make it go away. You can worry about your long-term mental health when you're NOT having an episode.)
Some psychoactive drugs (NOT alcohol) decrease dissociation in the long run, although they usually increase it in the short term.