How does ARRIs exposure index (EI) work?

[silvan schnelli]

I have read ARRI's whitepaper on dynamic range and how the EI is used to assign which signal value we want to choose as middle grey, which ultimately allows us to control the exposure latitude around middle grey. I've also read that the way ARRI does this is by only allowing us to shoot at its native ISO 800 (or maybe as some say its 400) and that changing the EI is like adding an exposure LUT on our image. This would make sense to me, because it would explain how the dynamic range (SNR=1 to full well capacity) is able to stay constant at different EI, because it doesn't operate by increasing the gain of the signal. (It would also explain why its they use the nomenclature EI)

Nevertheless, I would like to know how they do this and if what I have read is correct?