Perhaps I just have a bias against parallel dimensions (as evidenced from my interpretation of Silent Hill's otherworld), but I want to take some time to clear up what might be a mis-conception in the conventional wisdom interpretation behind Dark Souls' multiplayer summoning mechanics. Dark Souls co-op is not necessarily based on parallel dimensions, as many players seem to assume. It might, in fact, be intended to be an abstraction of some kind of time travel. I've noticed that many players online already seem to refer to the multiplayer mechanic of these games in terms of time travel, but I've yet to see any wikis, lore videos, or blogs that seem to explain multiplayer as a time travel mechanic.

I want to preface this analysis by stating that I'm not asserting that the following explanation is the absolute, 100% correct interpretation of the mechanic. Individual players may disagree based on their own reading of the game, and I'm personally somewhat conflicted on the topic myself. I merely want to propose this as a possible alternative to the defacto "parallel worlds" interpretation. I'm going to point out in-game evidence that supports the idea that Dark Souls' multiplayer is based on time travel, but there is also in-game evidence and mechanical evidence that flat-out contradicts that interpretation. I will address those contradictions as well. So that being said, please keep an open mind, and enjoy the read!

The summoning mechanic

There are two games in the series that are not part of the Dark Souls franchise, and which have different in-game explanations and rules for the same multiplayer features (more or less). Those games are, of course, Demon's Souls and Bloodborne. Both have asynchronous multiplayer and summoning mechanics that work similarly to Dark Souls.

Demon's Souls summoning operates under the idea of summoning the spirit of a fellow adventurer who's soul is trapped in the Nexus. This is why you must be in soul form in order to be summoned. Bloodborne's beckoning operates [similarly] under the principle of manifesting hunters out of dreams (which seems to operate under a similar cyclical paradigm to Dark Souls, but I'm not 100% sure). In Dark Souls, you aren't necessarily summoning ghosts (as you do in Demon's Souls), since the undead in Dark Souls are more akin to zombies than ghosts. Also, characters in Dark Souls can leave summon signs whether they are hollowed (dead) or in human form (revived), which is a significant alteration from Demon's Souls. A lore reason for summoning is provided in Dark Souls:

Solaire explains to us how summoning works:
"We are amidst strange beings, in a strange land.
The flow of time itself is convoluted; with heroes centuries old phasing in and out.
The very fabric wavers, and relations shift and obscure.
There's no telling how much longer your world and mine will remain in contact.
But, use this, to summon one another as spirits, cross the gaps between worlds, and engage in jolly co-operation!"

Both Solaire's dialogue, and the White Sign Soapstone (along with other online play items) make references to other "worlds", which leads to many jumping to the conclusion that each player's game is a sort of parallel universe within the Dark Souls lore. However, this may not necessarily be correct. Both Solaire's dialogue and the soapstone also provide explanations for these worlds: "time is convoluted | distorted". This seems to be the explanation for what is meant by "worlds", and it seems that Solaire and the in-game descriptions may be using "time" and "world" interchangeably (could it be a translation / localization issue?). The phrasing in the white soapstone's description joins "the flow of time is distorted", and "the White Soapstone allows undead to assist one another", into a single, compound sentence, which definitely implies that the two phrases (and ideas) are linked.

"Online play item. Leave summon sign.

Be summoned to another world as a phantom through your sign, and defeat the area boss to acquire humanity.

In Lordran, the flow of time is distorted, and the White Sign Soapstone allows Undead to assist one another"

 

The dialogue of Saulden (the Crestfallen Warrior of Dark Souls II) is even more explicit...

Table of Contents

• What the hell does poise do?!!
• Show the entire upgrade path for weapons
  • Filter Soul Transposition list by boss soul requirement
  • Show all the players equipment
  • Restrict summons' usage of the Storm Ruler
• Less arcane NPC quests
• Don't teleport the player from one boss to another!
• DLC suggestion: The Age of Dark
• Lifegems or Estus from message rating
• A method to the madness
  • Cumulative covenants
  • An actual sin system and restoration of the Darkmoon covenant
• Un-tipping the invasion scale
  • What happened to the smooth online play of DSII and Bloodborne?
  • Even out the penalties for invasions versus cooperation
  • Disable "friendly fire" for Red Phantoms

What the hell does poise do?!!

In my review, I noted that poise seemed to have been turned off in the game's code. Well, FROM Soft has apparently stated that poise is working as intended. Really? How? What does it do? FROM was not forthcoming (so far) with any details on what the stat is supposed to actually do, other than to say that it is "more situational". Ok, whatever. So I guess it's up to the community to try to figure out how this stat apparently works, since it doesn't work in any way comparable to the previous games, nor does it seem to solve the problem that the original implementation of poise was intended to solve.

UPDATE Nov 16, 2016:
It looks like we've finally figured out what Poise does in Dark Souls III. It seems to only be activated when using large weapons that provide Hyper Armor.

I had previously believed that Poise functioned as an "escape method" from quick, stun-locking weapons (like daggers). My early interpretation of Poise was that a higher Poise value may allow the player to escape from a stun lock and be able to roll / counter-attack / parry after 3 hits rather than 5 or 6 consecutive hits. I even remember testing this hypothesis out and finding it to hold true. This would also apply to situations in which a player gets attacks from multiple enemies simultaneously (or in quick succession). Higher Poise would allow you to escape from the 3rd enemy's attack, rather than the 4th or 5th. But according to the Wiki, this doesn't seem to be the case. Am I wrong?

If it is true that Poise only affects Hyper Armor, then I'm still not happy with the mechanic, as it only applies to very specific builds, and might as well be a stat on the weapon rather than a feature of armor. But if it does also affect the ability to escape stun locks, then I guess I would be a little bit more satisfied.

Poise was originally intended to act as a counter to extremely fast weapons like daggers, rapiers, and so on, that could quickly hit and stagger an opponent and put them in a stun lock from which they couldn't escape (so long as the attacker still had stamina). It was also intended to give players with slower weapons an opportunity to tank their way through hits with such fast weapons. You'd still take damage, but assuming your attack with a stronger, slower weapon did more damage than your opponent's weaker, faster weapons, then the trade-off would still be in your favor. If you were going to use a very slow weapon, then it behooved you to also equip heavy armor and other poise-boosting equipment so that you could tank through opponents' hits. That doesn't seem to be the case anymore. A knight in full heavy armor should not get stun locked in a 10-hit combo from a dagger. It simply shouldn't happen. This is what poise was designed to prevent, and it's not doing its job. If an invader shows up in your world with an estoc, and you aren't an expert at parrying, then you might as well just offer up your head on the chopping block and get it over with.

Without poise, heavy armor is worthless, and daggers are incredibly overpowered against slower combatants.

Without poise in this form, heavy armor simply isn't worth its weight encumbrance, especially since you can't even upgrade it to increase its damage resistance. This was a problem in Demon's Souls, which didn't have poise. Heavy armors generally didn't reduce damage enough to be worthwhile, they prevented the player from effectively rolling, and they were so heavy that they prevented the player from being able to pick up all the loot in a level because that game also had an item burden. Rolling was the end-all-be-all of defense in Demon's Souls, and that was one of the game's greatest weaknesses. It was easy to overlook because the developers didn't know better at the time. But they do know better now. Was poise exploitable in Dark Souls? Sure. It was really exploitable in Dark Souls II due to its connection to hyper armor and the inclusion of farmable healing items. But whatever FROM Soft did to it in DSIII seems like a severe overreaction.

Once I learned that poise wasn't working the way I expected it to, I gave up on trying to engage a lot of enemies with my slow halberd. Instead, I started fighting the knights in Lothric Castle and the Grand Archives with my flame-infused barbed straight sword - which I also spent a bunch of titanite to fully upgrade. This speedier weapon allowed me to attack these knights as fast (or faster) than they could attack me, giving me enough of an edge to reliably beat them. The ones with the tower shields and lances still gave me trouble, but the swordsmen fell swiftly to my +10 Barbed Flame Sword. Even Yahtzee noted in his Zero Punctuation review the starting long sword seemed better than any of the numerous boss weapons that he found. I imagine that this is because he also had trouble with the lack of poise, but didn't quite figure out that poise wasn't working.

Lack of proper poise allows the estoc's reach and speed to make it a deadly PvP weapon.

So this leaves us with the question of "what, exactly, is poise intended to do?"...