What IS Soulbursting?

I don't have the Fracture of Biel Tan book yet, but can anyone shed light on what is actually happening fluff-wise when our units perform Soulburst actions? Is it that our units are just moving really quickly but otherwise the old fashioned way, or is it more mystical than that?

Haven't got the book, but I think it's Ynnari units can gain power from souls around them (in-game from the recently deceased) and move in unnatural ways.

I can't tell whether it's a benign 'the souls help them along' style thing, or a more sinister 'they drink the power of the souls' style thing.

I hope it's the latter from a fluff perspective. Gives other Eldar good cause to dislike them, and the more fractious 40k is the more opportunity for good storytelling. Plus, there's a kickass diagram from one of the old Eldar codices where there's a truly abhorrent rune-concept in the ancient Eldar language which basically translates as 'Soul Drinkers', but is separate from the rune for Slaanesh.

Makes me wonder whether the Ynnari have rekindled the ancient horrific practice of drinking the souls of other Eldar...

Ah the wonders of headcanon

Well it goes without saying, but spoilers ahead

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Basically, soulbursting in the main body of the fluff is only told from Yvraines perspective, but it seems to act something similar to phoenix lords, or exarchs, when a nearby Aeldari is slain, she "inhales" their spirit, getting a new voice in her head, along with all their memories and emotion, and a burst of refreshing energy to knit together wounds. While she does muse if she could teach this to other followers of Ynnead, its never explicitly stated how it works for others, aside form her. The closest we get is the flavour text from the formations and psychic powers.

In the flavour text, its simply put that Aeldari draw on the swirling life energy being released by dying creatures in the battle, Aeldari or otherwise. For any Aeldari spirits they absorb, they add it to the collected bank of all previous souls they have absorbed, becoming like living infinity circuits. Doing so energises both them and the spirits they take in, both receiving a true "rebirth". For any other type of creature soul, they are merely an energy source, used and consumed for a burst of speed, quickened reaction times or what have you

Hmmm, i kinda hoped they'd leave it ambiguous and the sort of have. With things like this i always feel that it's best to leave it up to the reader's interpretation so they can decide for themselves based on how grimdark or noblebright they want 40k to be...

I always considered it akin to... la petite mort.