This seems not to be always the case.
I've observed that at a low SOC level (battery indicator at 4 bars), you can push as much as the half of the ECO zone, and still charge the battery.
Look at this chart, where S1 starts with SOC > 50%.
As soon as the Hybrid System Indicator raises over the second mark, Battery Current goes positive, thus draining power from the battery to push the car.
This is the commonly observed behavior.
Now, look at this chart, where S1 starts with SOC < 50% (4 bars on the dashboard).
Even going with the power meter at half of the ECO scale, current remains negative (flowing from the motor/generator to the battery pack).
So, where is this additional current coming from?
Engine RPM are the same in both cases:
Let's check Load and Fuel Flow:
Same values in both scenarios also.
This scenario has been investigated on a Hybrid Synergy Forum thread.