As efferman commented to the previous post, there is a new video showing how the PF Servo turns proportionally if controlled with a PF Speed Remote (8879) - The same if connected to the PF Rechargeable Battery (8878).
The PF Servo turns 90 degrees in both directions. See below.
As I also commented to the same post it works as per the known LPF RC Protocol, which is public.
With a PF IR Remote Control (8885) it goes full left/right when the sticks are pressed forward/back (depends on polarity switch). When there is no signal arriving it returns to the center like a regular servo. It behaves similarly when attached to a regular PF Battery Box (8881, 88000).
Now if connected to the PF Speed Remote or the PF Rechargeable Battery it receives pulse width modulated signals (PWM) while attached to the PF IR Receiver (8884) or the Battery, which provide proportional control, and it turns proportionally according to the received signal. That's why it does not return to the center automatically. The Stop button must make it return to the center though.
Hope it is clear now!