Ok, I read up on this for a while one time, and here is how I am pretty sure it works (at least in my 2013).

S-Flow off: Heat/AC functions as you would normally expect it to in any car without an S-Flow-like function

S-Flow on: S-Flow can sense driver and passenger seat. S-Flow cannot sense rear seat occupants. Thus, you will need to disengage S-Flow when you have rear seat occupants if you want their ventilation to function normally (again, this is for my 2013, so they may have changed it in later models). When you first turn the car on, the Heat/AC system will use all vents (front, rear, floor, dash, whatever) to bring the cabin to your desired temperature, working as though S-Flow is not engaged. As it approaches/reaches the desired temperature, it will switch to only the vents for the driver, and passenger, if passenger is in the car. I think what I read claimed S-Flow could have fuel savings of like 4% or something (probably only with AC in the summer?).

I use S-Flow, because why not? It does a good job of keeping me comfortable. If it didn't, I wouldn't use it.

I should mention that I rarely have rear seat passengers. I'd be less inclined to use S-Flow if I had rear seat passengers more often. Nice to have their ventilation working (especially since I have the lux package and they have their own controls back there) without having to fuss with S-Flow.

 

The other benefit to S-Flow not mentioned is if the passenger has left the temp considerably different than the driver, i.e. driver at 70 passenger at 78, the system only uses the driver settings to control the temp when the light is green and a passenger is not present.

 

True! Did not think of that benefit, but that aspect has definitely come in handy for me. I never understood why our cars didn't have a "sync" function for the climate control like a lot of other cars do, but that could explain it.

 

I'm pretty sure the "Sync" function that other cars has is achieved in the GS by turning the "Dual" option off.