Basing on what is happening from the panels and the mangastream translation:
It's not simply luck, since he states that the scales of battle (and not luck) must be balanced:
which means, the reason why he tries to kill them is to balance the battle (though he might be using the balance of karma in order to punish them).
Then Hashvalth mentions that they are fortunate, because he longer has a reason to kill them, since as you can see, the Scale of Battle have titled in the Shinigami's favor greatly (the Quincy side jumps up, because of the sudden weight added to the Shinigami side, the Shinigami side isn't showed though, but it's pretty easy to deduce what's happening basing on what's going on with the Quincy side):
at this point, the Quincy are at a great disadvantage, reducing the Quincy force might be even risky at this point.
Also, the Scale of Battle is something that Hashvalth can show or it's perhaps even a physical thing residing within their HQ (quite a big tool :P), since Cang does see it, shocked as to who would be capable to tilting the balance so much by himself? (especially seeing as the Quincy had advantage just a moment ago) ? Seeing as at this point, Renji and Rukia, neither Byakuya or Ichigo, haven't yet entered the fight, the only person left that could do that to the scale, a Trump Card, a Game Changer, a Balance Breaker, is one and only Zaraki Kenpachi... unless Urahara finally released all of his forces :P (interesting enough, it also seems that he is keeping a look at the Balance, since he entered by himself...).
Seeing as killing a Quincy willingly at this point would actually be a problem, as it would tilt the balance even more towards the Shinigami (that already was heavily titled toward them by someone), the one that killed I-ritter couldn't be a Quincy, but an enemy (plus I-ritter already showed his power, so Kubo didn't need him anymore :P, while K is probably still alive).
Then there is this manner of cutting through something of superior defense:
Really, the only people left with such superior cutting vertical slash techniques are Zaraki, Grimjow and Isshin (he usually uses GT then). On the other hand, Hashvalth aims at weak points (like a neck, just like he at first tried to do) and use appropriate amounts of power to cut down his targets (like that Shinigami with glasses, that had a deep wound across his body, but wasn't split in half, while Cang was split in half).
Also, it's worth mentioning that Cang Du might not be dead, since it looks like it was his arm that was mainly cut off (so again, simply cutting someone's arm sounds like a strange way of execution...). The reason why the vertical slash even cut off only his arm, might because he realized that he has to avoid (the incoming slash) it (so if Hashvalth couldn't cut him, while he wouldn't avoid the slash from Emperor, then from whom could it be coming ?).
No, he clearly states in the next panel that a life saved by fortune must also be balanced with an equal amount of misfortune. By saying that the scales of battle must be balanced, he is not talking about souls or lives in battle, he is saying that in battle, fortune and misfortune must be balanced. Quincy have never been concerned about keeping the balance of souls, hence why they eliminate hollows, why would they start caring now?
Also, I think fortune in this instance is a little more than luck as one's own personal skills can factor into it besides just pure chance, such as in Cang Du's case. Haschwald's (or his zanpakuto's(?)) ability seems to react to fortune in battle and can tip the balance as he had stated to Kyoraku earlier.
That's the mystery ;), since there is no purpose to it for Shiningami?, yet there is one for the Quincy.
Also, the one that first used the word "Fortunate/Fortune" was Yhwach (the one that even says "be grateful for that fortune" might have been Yhwach as well), so Hashwalth simply continued the notion of his speech (by continuing it with saying "However... the scales of battle must be balanced." and then he uses the "fortune" word since it was already used by the Emperor to describe their predicament).
The words that only came out of Hashwalth (not being mentioned by Yhwach) were "Balance" and "Scale", the notion of putting his Sword before himself as if he was scaling something is also important, which all of it might refer to his power. Also, Hashwalth used the word "Scale" only when referencing the "Scale of Battle" so naturally, the gigantic Scale that appeared was showcasing the scale of Battle (that's why on one side of the scale there is a Quincy Symbol).
If Hashwalth's "B" is really "The Balance", then his power might posses many applications. For example, regarding Ichigo's Bankai, if his Bankai was scaled with 30 Attack and 70 Defense, then Hashwalth might have tipped the Balance (as he himself mentioned it being his purpose) the other way around, thus making his Bankai being only 30 in defense, or simply taking the 70 points in defense and putting them to his attack (thus being 100), thus changing it's defense to 0, which would mean that cutting/destroying/breaking it upon impact would have been a child's play.
This stuff is way too complicated, and has little or no point to it at all. This is a war, and the Quincies were supposed to anihilate the Shinigamis as quickly as possible. What's wrong with Juha Bach letting Cang go back and do just that or die in the attempt.
Jst what is Kubo's point/purpose.
Cool chapter. I like that they are bringing the balance thing back again, I think the worlds will collide or something will happen in the precipice world if they mess the balance up of how many souls get reincarnated and how many get wiped out of existence. Cant wait to see if Rukia and Renji and save face after their miserable showing last time they fought the Quincy. Those two talk so big with such assurance yet always get their asses handed to them.
I certainly hope they killed those 2 sternritters because Renji and Rukia showed up and their power was so much greater that they had to balance out the field again. I doubt it could be that they just 'lost' to the captains and Bach was pissed, at least I don't think it would be that way. Or the only way they can stay in SS in such a way is by having a balanced field of Quincy to Shinigami or else the SS world would reject them back to the shadows, like kicking them out of the dimension. But what will they do when Ichigo shows up hmmm. Also I hope Byakuya is last to come down, kind of to save the day, if not im pretty sure he just went down ahead of everyone and snuck into SS without anyone noticing, keeping with his stealth persona. Maybe fear Quincy shows up to deal with Renji but Byakuya just shows up out from the shadows to handle his revenge before Renji can even both pulling out his sword heh.
It's worth mentioning that the "Scale of Battle" and "Balance of Souls" might be two different things. Balancing the Scale of Battle might be some kind of ritual or preparation in order to do something.
From what I understood, since the Balance was disturbed, they chose to cut down their failures, instead of sacrificing those that can still be useful. Also, there is a certain notion that all of the SR have so far, they are not prepared to die in battle, but rather prepared to be killed by their Emperor. It's like they know that they are going to die anyway, that they are going to be sacrificed for a "greater" purpose. The whole point of balancing the "Scale of Battle", might be so that the numbers on both sides are meant to be reduced equally, so that only 2-3 Qunincy are left, which have accumulated all the powers of those that died, while at the same time reducing the number of Shinigami.
Then one has to wonder if the reason why Aizen haven't just slaughtered everyone on the way, is perhaps the same one why Quincy don't do it as well ?
If my reasoning is right, then when Hashwalth mentioned that the second show of fortune had befallen the I-ritter and K-ritter, he meant that they no longer have to sacrificed for the time being, since it wouldn't help the balance (which was the purpose of cutting them down in the first place according to Hashwalth). While, if I understood correctly, Squad believes that the fact that Cang Du had survived the first slash from Jugram, to be the second show of fortune.
My question is who got Cang out of the ice prison....? And Soi Feng dead now? Where is Bazz B, and do the fullbringers, and hollow's balance out with the Shinigami and Quinices? Where is Aizen at in all of this, and I want to see Isshin fight.
Seeing Renji and Rukia now, it feels like the Robes surrounding their bodies may have been designed to offer some resistance to Quincy Reishi manipulation techniques.
Pure: I think Cang got out of the prison when he went SSJ and it overtook the prison, but then again Hitsugaya's "1000 year prisons" have been kind of weak ass anyways, I don't think we have every seen anyone actually held/frozen in one permanently yet. Rukia actually managed to do more with her zan than him I think. Soi Fon probably got away from any serious attacks with Shunko activated so Im betting shes alive just tending her wounds somewhere. And the 'other' forces Im assuming just become regular souls when they died, just souls with extreme reiatsu, not Shinigami or Quincy but just souls caught in the balance/crossfire. They are probably going to be too small to matter to the balance. The Quincy might be alive still since as far as we know Uryuu's soul chain is still intact and so he can't leave his body like Ichigo can. Im thinking the other Quincy fighting are all alive too, and that they need to have either a special condition met or a correct balance of quincy/shinigami powers around in order to hang out in SS. It doesn't make sense that living people can just hang out in SS, a place for souls with no physical bodies, and be okay, I bet there is some kind of time limit or consequence for staying there too long. Which would be kind of weird for HM too but idk, im getting too far off course. Maybe thats what the balance is, living quincy to dead shinigami, fuck it up too much in one direction and the whole SS realm will eat itself lol