Cine EI is a mode that forces you to pick either of your camera’s dual base ISOs and gives you a way to then lower it on the screen to see what it will look like in post once you adjust the exposure.
So you are recording a slightly over exposed image in order to get the highest amount of detail in your shot, then in post, lowering that exposure.
Once you have picked either ISO, you gain the ability to preview what it will look like in post by selecting an exposure setting that will only be applied to the monitor, not the final image. https://www.youtube.com/watch?v=_ZO9wqwjd0E
For Sony cameras in cine EI mode, a lot of DOPs will:
Lower ISO = more shadow dynamic range
Higher ISO = more highlight dynamic range
If you put your camera into Cine EI mode, you can choose your base ISO of either 800 or 2500 (for my FX30).
If you were shooting a day scene and wanted lots of detail in the highlights, you should set it to 2500 and then lower the EI.
And if you were shooting at night, you would lower the base ISO to 800 and then lower the EI.
Because of this, DPs will often make the ISO a high number when shooting sunny scenes because it gives them more dynamic range to play with in the highlights when grading.
And contrastly, will make this ISO a lower number when shooting darker scenes so the shadows look cleaner/less noisy. But you would need more lighting to achieve this well.
https://www.youtube.com/watch?v=ZOqExwwyCBY
(I need to do some test shoots to confirm this information myself.)