It's not a "valve" per say, it's actually a pressure regulator that has an outlet pressure gain set by the geometry of the internal components (i.e. the 57% pressure gain you get on the outlet vs. inlet above the knee point).
This has some information on how it functions, and I'd assume it's a single stage regulator due to the size and application:
https://en.wikipedia.org/wiki/Pressure_regulator
At least that's my understanding of how they work without cutting one open, but it seems consistent with their operation. You're right in that it can't be something like a needle valve, as that only throttles pressure based on pressure drop vs. flow.
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