Slaanesh still hungers and takes their souls, but if another being is being tortured Slaanesh goes after that ones soul 1st and no longer touches theirs, it is said torturing others makes them feel like their souls are being rejuvenated (From what i read this is the wording they used, and i think its not really rejuvenating them, but kinda like when you take your shoes and socks off, that feels good, feeling of relieved, their souls where being pulled at and now they arnt).
How does Eldar stop the pull? Well with Spirit Stone. Spirit Stone are crafted for each user, what Yvraine did (Yvarine being Born as Biel-tan knew how to make Spirit Stone) she makes all her followers have Spirit Stone and additional ones (void of personality and not crafted for each person) to make them stronger when another falls.
So.. basically, its just Spirit Stone.
Edit: I want to add, that Spirit Stone are really hard to get, you have to go into the wrap to get them. So i honestly dont know, but am guessing (I still havent read all the Lore for Ynnari yet) that she is taking Spirit Stone from the thousands of dead Biel-Tan that helped her make the Yncarne.
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AoS: BoC 9k, CoS 3k